I was told by my teacher that a triplet will always have a three written on top. I've also seen this in theory books for graded exams which have also said that any triplet grouping will have a bracket around it. And I've read in other places that any tuplet will have a bracket over it. But I'm playing Moonlight Sonata at the moment in which there are no brackets or 3's written. I wonder if it's for the clarity on the page given that the triplets play all the way through. Would appreciate any feedback. Do some versions of Moonlight Sonata have the three written? And is it actually quite common for pieces with triplets to not have the 3 written?
Yes, in theory exams give all your triplets a number and a bracket.
And yes, in 'real music' when a triplet figure becomes established it's common to omit the '3' after a few repetitions, or even from the outset. And 'Moonlight' is probably the most-quoted example of this happening!
Moving into the Romantic era it became quite common for (piano in particular) composers to write 'impossible' rhythms, sometimes using small noteheads 'quasi cadenza' but very often not. It is generally easy to see what is intended, both on a 'fit these notes against these notes' basis and with the knowledge that the performance was intended to be flexibly rubato, not strictly metronomic. (Though it's often interesting to see what happens if the beat IS rather less flexible...)
Later still, composers started writing complex tuplets that (I think) ARE intended to be played with precise rhythmic accuracy.
It's all about context. And fashion.
Normally a triplet is marked with a '3' and a bracket, mostly on the stem side, but often on the notehead side. If the triplets continue for a long time and it's obvious where they are, some editions leave out the marks after the first couple of triplets. Very occasionally an edition will leave them out completely: in the 'Moonlight' Sonata it's very obvious they are triplets.
The bracket can be left out if the beaming makes it clear which notes belong to the triplet.
The same applies for any other kind of tuplet.
When reading older scores there are various ways to notate triplets: brackets, curved line,
3 in parenthesis. If memory serves, I've also seen old scores with groups of three eighth notes in simple, duple meter. I assume that lack of special tuplet notation meant performers understood to play them as triplets simply from the beam grouping. Also, the placement of the tuplet notation may be above or below the notes.
Like many things in notation, tuples are used to help make the notation clearer. It is likely that the time signature of the music you're reading is 12/8. Thus,the intended/implied subdivision of each bar (12 eighth notes) is four groups of three eighth notes. In other words, the eighth notes are played at "face value" in 12/8, and so may appear without a tuple marking.
Contrast this to a case of 4/4 time, where the implied subdivision is 8 eighth notes per bar. Three eighth notes would consume one and one-half beats at "face value," but when those three notes are beamed with a "3" tuple, this tells the reader that the three notes are to be played in the normal timespan of 2 notes, i.e., 3 eighth notes in one beat.
Conversely, in 12/8 time, you may sometimes see two eighth notes beamed with a "2" tuple, to indicate that those two eighths should occupy the time of three.
In general, one might think of it in terms of what is the rule and what is the exception. In 12/8, it is generally the rule that 3 eighths form one (subdivided) beat, and so the rather redundant triplet notation is omitted. Instead, the exceptional case of 2-to-the-beat eighths are marked with a "2" tuple. In 4/4, the reverse is true: the conventional 2-to-the-beat eighth notes carry no tuple marking, and a "3" tuple is applied to triplets that are intended to be 3-to-the-beat.
Clarity in notation is not always achieved by being explicit. If a score has triplets everywhere, then repeatedly writing a "3" everywhere just leads to clutter and annoyance. The "Moonlight" Sonata is one such example.
Sometimes, the n-tuplets are implicit even when they aren't repeated. For example, in the opening of Rachminoff's Piano Concerto No. 2, there are sometimes 8 notes to a beat, sometimes 9, but they are all written to look like eighth notes, even though sixteenth notes would be a better representation. You just figure out, based on context, how to squeeze the required number of notes into the allotted time.
No not all scores have it indicated with a three. Some have it indicated with a three some do not. Some use a bow. Some use a bracket some scores have no indication of triplets at all and just have three quavers in a crotchet beat that you assume are triplets.
Ultimately, the score will be notated the way the composer and publishers want it to be notated.
Although there is a tradition of what is good notation composers can and often do disregard many things that are consider "good" or "proper" with just about anything they want.
Whether to place a "3" and a bracket (or some other designation of the grouping) above or below a triplet was a decision made by the composer (in the manuscript) or by the editor (in printed music).
In the case of the "Moonlight" sonata, other elements of the notation make it clear that the eighth notes are triplets. These include
(1) the beaming of the eighth notes in consistent groups of three and
(2) the combination of the alla breve time signature (so-called "cut time") with measures containing twelve eighth notes (or, for that matter, three eighth notes per beat, since the vertical line-up of the notes in all voices should follow the meter: all notes that begin on beat 1 should be lined up and likewise for all the beats and their subdivisions.
You can view Beethoven's own manuscript here:
Be aware, though, that the first page of the manuscript is missing from the pdf (if not also from the actual document). Anyway, you'll note that Beethoven doesn't write the "3" for the triplets, but it's clear from the beaming and the context that that's what he means.
We also find the triplets missing from the first edition from 1802 (https://s9.imslp.org/files/imglnks/usimg/3/33/IMSLP51038-PMLP01458-Op.27-2.pdf).
For printers, two reasons for suppressing the "3" in cases like this were economic: to save ink as well as the expense and time of typesetting a "3" over each group of notes.
On the IMSLP site, you can view a number of editions that suppress the "3" as well as some in which the editor likely intervened so that the notation would appear formally correct. That decision may have also been dictated by the house style of the typesetter or publishing house.
Keep in mind, too, that using the "3" throughout would make the page look terribly cluttered, and for no other reason than to be formally correct.
Sometimes it's clear from repeating notes that triplets are meant. I was looking at scores of Bach's famous Toccata and Fugue in D minor and some scores omit writing out the 3 above triplets, even when next to regular (duplet) note values.
Examples from IMSLP:
Johannes Ringk (ca. 1740-60):
Wilhelm Rust (1867):
Pierre Gouin (2007), now with the first 3 written out: