Why don't we see, for example, a clef with a stylised symbol representing an A, B, D or an E? What's so special about G, F and C? Who decided to use them and why?
There is a lot of speculation in other answers, but this question actually has a very specific and exact historical answer, at least regarding the F and C clefs. To summarize, the basic answer is that Guido of Arezzo decided to use F and C as anchor notes in the early to mid 11th century, mostly to indicate that these notes had a semitone below them. The G clef was added much later, to provide an anchor note in the higher range.
To answer in more depth:
What's so special about G, F and C? Who decided to use them and why?
The "who" is Guido of Arezzo, the person who came up with the musical staff (among other things). Before Guido, there were symbols called neumes that indicated the general shape of a melody, but there was no way to indicate precisely how high or low a particular note was in relation to the neumes around it. The best that scribes could do is write some neumes a little higher or lower than others. But there was no staff, so these neumes are sometimes referred to as in campo aperto ("in an open field"). (See example here, with neumes written above the words.)
Guido proposed a method around the year 1030 to measure precisely where a note was in relation to others:
The notes are so arranged, then, that each sound, however often it may be repeated in a melody, is found always in its own row. And in order that you may better distinguish these rows, lines are drawn close together [...]. All the sounds on one line or in one space sound alike. And in order that you may understand to which lines or spaces each sound belongs, certain letters of the monochord are written at the beginning of lines or spaces.1
Those "certain letters of the monochord" meant the note names A, B, C, D, E, F, G. Actually, B came in two varieties at this time: hard b or square b which we now call B-natural, and soft b or rounded b which we now call B-flat. Those eight possible note names/letters were the complete set of standard notes at the time. This is the origin of the musical clef, as the F, C, and G signs we still use are stylized versions of those letters. Originally, in Guido's system, any note letter could function as a clef sign by marking a line or space.
But since Guido's notation system was so new, he added yet another guiding feature -- colored lines:
And the lines are also gone over in colors [...]. We use two colors, namely yellow and red, and by means of them I teach you a very useful rule that will enable you to know readily to what tone and to what letter of the monochord every neume and any sound belongs; [...] wherever you see the color yellow, there is the third letter [C], and whenever you see the color red, there is the sixth letter [F], whether these colors be on the lines or between them....2
Originally, F and C were singled out and indicated with colored lines. Note here that these colored F and C lines could actually occur on a staff line or in a space, and they could be indicated in any octave. Why did Guido choose these particular notes? Two main reasons:3
- F and C were the two places in the scale that had a semitone below them. It's important to remember that even up until the early 1600s, it was very common for clefs to move around even from line to line in a particular written part. Singers needed to be able to immediately reorient themselves to where the notes of the scale fell on each line. If you're not used to our modern practice of having clefs fall on a particular line, the main thing you need to know is where the semitones are. Since medieval chant originally was not notated with accidentals (other than the potential B-flat), every other line/space other than C or F could usually be assumed to denote a whole-tone relationship with the notes around it.
- F and C tend to be sort of "anchor notes" in chant notation. Even at this time, many melodic figures made use of a kind of leading tone that would resolve up by semitone to F and C. Thus, chant notation tended to favor F and C as the important notes in certain neumes, so these notes were in some ways more prominent within the scale. (This explains the choice of F and C to indicate the semitone, rather than E and B to indicate a semitone above.)
Here's a 12th-century example of this early notation, with red lines to indicate F and yellow lines (somewhat faded) to represent C. You'll also notice the F in front of the staves on the upper part of the page (which looks a bit like our modern F clef with its two dots forming the lines of the letter F), as well as the C notated in front as a clef on the bottom of the page.
As noted above, in the first Guidonian notation any letter could be placed in front of the staff to indicate the note for a line or space. Looking at the first couple centuries of staff notation, the most common pitches singled out with a letter/clef were, in descending order of frequency (and using modern octave notation): F3, C4, F4, C3, D3, A3, G4, E4, G2, B2, B♭3.4
The most common notes for clefs were F3 and C4 (what we now call "middle C"), where our modern clefs tend to be located. But there was a great variety of notes used for clefs. On the other hand, the use of colored lines was much more restricted. Very soon after staff notation was introduced, it became common to put both F3 and C4 on lines and to color them red and yellow respectively. While one might see other F and C lines in other octaves (on lines or in spaces between the normal staff lines), the most common ones came to be on F3 and C4, likely because they fell closest to the middle of the range of notes where most chant melodies were notated. (Further proof that these colored lines were used to indicate semitones below can be seen with the rare but somewhat consistent use of green lines to indicate the one possible accidental of B♭ in some early manuscripts. In this case, the C line was sometimes not colored, because it wouldn't have had a semitone below it but rather B♭ instead.5)
In any case, eventually the function of the F and C colored lines was combined with the letter notations of clefs. Originally, the non-colored lines of the staff were often merely scraped into parchment or colored only lightly. But changes in pens and ink over the centuries made it easier to draw many long lines on a page, leading to standard black or red lines for all the lines of the staff. The cumbersome practice of having to color some lines red or yellow (for F and C) was gradually phased out. Instead, F and C emerged as the standard places to put clef signs, now on a staff of only one color. As F3 and C4 were central to the range of most chants and they both occurred on lines (which had previously been specially colored) these particular octaves of F and C became standard locations.
The G clef became standard much later. In general, there was a need for an anchor point in the upper range of the scale. The standard Guidonian scale (or gamut) ran between the notes we now call G2 to E5. A G2 would only require reading three lines below the notated F3 clef, but a note as high as E5 would be more than four lines distant from the C4 clef. As the standard staff that gradually emerged in the late medieval period only tended to have four lines (though this continued to vary for centuries), there was a need to have some other standard note marked as a clef in the upper register.
Unlike the clear evidence from Guido explaining the origin of F and C as central points for clefs, there's no similar single source for the emergence of G clefs. However, there are several factors that may have played into the choice of G as the third place for standard clefs:
Simple symmetry: once the F3 and C4 pitches became standard places for clefs, placed two lines apart, it might make sense to place another clef two lines above C4, i.e., on G4. Many early staff diagrams in theoretical treatises show all the lines of the entire scale, with the central F-C-G placement displaying an obvious symmetry. (This might be confirmed by occasional instances of D5 also carrying a letter/clef in early sources, again two lines above the G4.)
Once F and C became the standard places for clef signs, G was already a prominent note to use for a third clef, particularly at the very bottom of the scale range on G2, a note then called gamma-ut. We still use that G line as the lowest standard line in modern bass staff notation, as it was the lowest note of the scale. Even in much later sources (up to the 16th, 17th, and even 18th centuries), this lowest staff line was sometimes marked with a capital Greek letter gamma (Γ), particularly in theoretical treatises explaining the origin of the scale. Given the central use of a G two octaves lower, perhaps the G4 line also seemed a reasonable place to indicate a clef.
G was also the starting pitch of a hexachord. Aside from staff notation, Guido also introduced a method of solmization, originally using the syllables Ut-Re-Mi-Fa-Sol-La. The Mi-Fa interval contained the notorious semitone. In the original medieval scale, there were only three places to begin the Tone-Tone-Semitone-Tone-Tone pattern of the hexachord: C, F (with a B♭) and G (with a B♮). As the two other choices for clefs -- F3 and C4 -- functioned as Ut in hexachords, one natural choice for a clef in the upper range might be G4, which also functioned originally as the highest Ut in the gamut. (As mentioned in the point above, the lowest Ut was already frequently signaled with a Γ clef.)
Simple process of elimination. Once F3 and C4 emerged as the standard clefs that fell on lines, a clef in the upper range should also preferably fall on a line. E4 is problematic as clefs originally indicated semitones below, and this one has a semitone above. B4 is similarly problematic and potentially contained the flat/natural issue too. D5 could work, and is in fact seen sometimes (as I noted above), but it's rather high in the topmost register of the original gamut. G4 is on a line and lacks the other issues, in addition to all the positive features I've already noted.
Whatever the exact reason (probably mostly points 1 and 4 above), G4 emerged as the other standard clef location during the 14th century and has remained so ever since. However, it should be noted that until the mid-18th century, the F and C clefs were still the most standard ones, the ones originally based on Guido and the position of the semitone. G clefs until the past couple centuries tended only to occur in notation for certain instruments that were pitched high; they were rarely encountered in vocal music or keyboard music (even with a wide span), where C clefs were still preferred in the upper range.
1From a Prologue written by Guido, quoted from Thomas Forrest Kelly, Capturing Music: The Story of Notation (Norton, 2015), pp. 62-63.
2Quoted in Kelly, p. 63.
3For a more detailed explanation, see pp. 18-19 of Joseph Smits van Waesberghe, "The Musical Notation of Guido of Arezzo," Musica Disciplina 5 (1951), pp. 15-53.
4Smits van Waesberghe, p. 33.
5See Smits van Waesberghe, pp. 41-43. For further discussion and interpretation, see Anna Zayaruznaya, "In Defense of Green Lines, or The Notation of B-flat in Early Ambrosian Antiphoners," in Ambrosiana at Harvard: New Sources of Milanese Chant (Houghton Library, 2010), pp. 33-56.
Three clef signs are in common use, plus the neutral or percussion clef.
The most common are those found in piano music, where the two clefs are separated by a space where 'middle C' lives, on its own leger line when it's needed.Go up a 5th from that C, which has become a sort of data point, and there's the G line, around which curls the G clef - often known as the treble clef.
Go down a 5th from C, and there's the F line, around which curls the F clef - often known as the bass clef. I guess those notes were considered pivotal, and they're also symmetrically placed.
A lot of instruments can have all, or most of their notes written within one of these two staves, with not too many dots being on leger lines. However, some instruments' ranges fall awkwardly, so the C clef is used. It's moveable, and can make any line into the place where middle C lives. Don't usually see it depicting middle C on a space.
Other clefs exist, for example a treble clef with an 8 attached, which gives a useful place for music for guitar. All the notes are actually written an octave above their sound on guitar, which saves writing it out in bass clef, and then having to utilise leger lines.
The reason there aren't A clefs, etc., is probably that the C clef, being moveable, will do pretty well any job that a (new sign) A clef might, albeit one note name out. So we already have enough tools for the job. And loads of musos are happy using them; they don't confuse; music is working well with them.
The neutral clef by the way, is for mostly untuned percussion, where, for instance, a snare drum is given the middle line for its dots.
EDIT: according to 'The Origin of Musical Clefs', the treble clef started life as a 'G', the bass as an 'F' - quite easy to see - and the C clef as a 'C', although why isn't stated. C clef's fairly obvious, but as to why treble needed to look like G, and bass F, I haven't yet discovered.
Almost any convention could be different and, in many cases, probably better as well. However, in most cases, one has won and it is too disruptive to change.
Of these clefs, probably the C clef is easiest to understand even though it is the least commonly used. It marks the position of middle C which has become a standard reference point. This clef is used where possible. It becomes unavailable when you go as far up as the treble clef or as far down as the bass clef as neither of these has a line for middle C. I guess that a variant of the C clef could have been used to mark the C an octave above (treble clef) or below (bass clef) middle C but these would need to mark gaps between the lines which might be less clear. The next most natural interval is a fifth and this has the advantage that a fifth above or below middle C is also on a line. So, marking a fifth above middle C (G clef) and a fifth below (F clef) is quite a good solution.
Edit: Some care is required with the word "clef". It is used in two slightly different senses: referring to the symbols (as you are using it); referring to a particular symbol in a particular location. For example, the "alto clef" and the "tenor clef" both use the C clef symbol but in a different location. Today, this is only common with the C clef symbol but it can occur with the G and F symbols. This article seems quite good: Clef (Wikipedia)
Edit: I forgot the percussion clef, see Tim's answer.
When first trying to show absolute pitch levels, European musicians did use a single line through some note-predecessor marks to represent a particular pitch. The earliest "clef" marked the mi-fa semitone (the only one if one considers octaves to start over at C rather than being connected to B below.) Other lines (and clefs) followed. The signs used (F,C,G) for clefs are basically a historical accident (in the sense of a random happening) though marking the semitone did yield a rationale. One could guess that were musical words written using a futhark, the clef signs may have looked different.
Clef does come from the root for "key" (the lock turning device, not the islands) as in the "key" to a puzzle.
What's special about G, F, and C, is that they are the keys in the middle of the system: C has no flats or sharps, F has one flat, G has one sharp. As such, they were the most natural notes to indicate.
Probably this is some sort of misunderstanding: While clefs were named according to their appearance, today this is far from obious. The note they are named after is of no special importance to that clef.
It is merely an indicator, where to put a characteristic feature of the clef (in case of G and F clef the inner center of the spiral, for the C clef its "nose"). It turned out, that the name is succinct enough to refer to the clef shape and so it stuck. Note, that only C clef maintained a variable position until today, therefore additional names were required like tenor clef (a C clef sitting on the second line counting from up to down).
The clefs, and their corresponding pitch location placements, have their historical origins connected to the ranges of voices and instruments, and the related problems of notating musical sounds. In particular, starting with the ranges of "typical" human voices, the frame of reference was based on the ranges (of basses, tenors, altos, and sopranos) that were considered "normal" for western art music and its predecessors. The point of the clef's placement is to provide a shifting window that ensures that the "bulk" of the notes (pitches) for a given piece of music and part (bass, tenor, etc.) end up on the staff, and not above or below it on ledger lines. The reasoning being that the pitches on the staff are easier to read and locate than those that end up on ledger lines above or below. It's as simple as that, really just a mechanical artifact of the notational system that was imposed.
Consider that if there was just one staff for all musicians and instruments, with all pitches on it absolutely positioned, that would take an awful lot of paper to write out the parts, and the bulk of that paper (vertically) would be "noteless" wasted space, with nothing but empty staff lines and spaces, no notes. The clefs (and the closely related situation of instruments being pitched in specific "keys") are really just a mechanism to solve this problem of notation. Not that we need to make it a contest, but with the exception of the C clefs being used for some relatively limited situations, one could say that the system of instruments being pitched in certain keys was a much more "successful" attempted solution since it persists to this day, while the use of clefs other than treble or bass has fallen off, noting that the overwhelming bulk of all music today is notated on either treble or bass clef (or the combo of the two).
There are lots of examples of instruments that, over the years, have moved around from staff to staff. Johnny Smith had a guitar method in the 50s and 60s that had guitarists reading off of the grand staff (same as piano) while current guitarists read treble clef although the instrument is transposing down 1 octave from where it is written, similar to the tenor sax, which is transposing down a major ninth (thus Bb tenor sax). Baritone horn (Euphonium) and tenor trombone can both have parts written either in treble clef Bb and transpose down a major ninth or they can read bass clef at actual pitch (in C), and if you play those instruments, you are prepared to deal with either.
Bach, the famous composer, originated our present form of notation. There is the treble clef (the "G" clef you mention), the base clef ("F" clef), and the movable clef, which can be tenor, or alto, or anything. Up to recent times, there were more clefs. For singers, there were four different ranges; soprano, mezzo, tenor, and bass. The "Grand" or great staff, with eleven lines, contains both the G-clef and the F-clef.
The reason they have survived to the present day, is that they serve the scales of most instruments. Even so, there are many instruments that use the movable, "C-clef." The "tenor" clef is used for the upper ranges of the bassoon, cello, euphonium, double bass, and trombone. The viola uses the "alto" clef. There is even a "sub-base" clef.
For a history of music, youtube currently is hosting 'Howard Goodall's Story of Music'. It's a six part documentary, that goes from Roman times, showing the origins of notes and scales, up to modern day.
Also, for a classic on notation and ear training, George A. Wedge's book - 'Ear-training and sight-singing applied to elementary musical theory, a practical and coördinated course for schools and private study' is free online, and very inexpensive on Amazon. Go to https://archive.org/details/eartrainingsight00wedgiala for a preview of it.
Hope this helped. Good luck with everything. B'H.