How do you know whether to use 1st, 2nd, or 3rd position in a guitar piece? How is it labeled in the music?
(Disclaimer: This is from my classical guitar experience. Maybe different styles do it differently.)
Sometimes, you would use a Roman numeral denoting the position, and draw a horizontal line above the notes that should be played in that position. A good example would be an image that came by in a question a couple of days ago:
Here, the Roman "I" and the horizontal line show what should be played in the first position.
However, most often, the position is not denoted at all. It's just yours to figure out. I see two reasons for that:
Putting labels like this everywhere in the score would just make it totally cluttered and harder to read, since (at least in classical) you switch positions all the time, while
Everybody would just ignore the markings, because there is either only one possible way to play the thing, and then it's not that hard for you to find it yourself, or there are multiple ways, and then you can be absolutely sure that each of the ways has some people who prefer it. Then, if you encounter a position marking which doesn't fit what you think to be best, you just grit your teeth and say "What a bunch of idiots the editors were, it's clear it has to be played in the way in which I play it..." :—).
(If you really want to see it in action, search for Etude no. 1 from Heitor Villa-Lobos on YouTube (it's only 90 sec piece), and have a look at a couple of guitarists playing it. There is one particular crazy run in the middle, and it seems that each guitarist has their own unique solution to that run.)
Finally, you can encounter these markings pretty often in the score, but they actually mean that a barré should be used on the given fret. Often, there will be a letter "B" or "C", so "C3" would be a barré on third fret, while something like "½BVII" is a barré just over three strings, at the seventh fret. (And in some scores, the "C" or "B" is just not there, so there is a potential for confusion.)
There are actually two questions in one here, and the labelling part has already an answer from a previous question. I'm looking at the other part.
Given that most of us have four fingers, so the fretting hand can encompass four frets, plus if pushed, an extra one lower and higher, we look at the notes to be played, and find a place on the fretboard where all the notes can be played comfortably, without stretching.
Of course, this means looking at the music in a different way than purely reading the notes, as on guitar, there are often several places to play the exact same notes. So knowing the fingerboard is very important.
If you're asking about classical guitar, sometimes, as mentioned in other answers, Roman numerals are printed to help the player. They tell which fret is a good place to be - barred or not. Barred usually has CV, or BV written above, although merely putting V is enough for some publishers.
If you're talking about other guitar playing, you have to work out your own, and that will be different for different guitarists. That's where 'proper' music scores excels over tab. Tab always says, "This is where it gets played" (whether that's the best place is often questionable).
There is a little ambiguity here. We can tell you how positions are labeled but that doesn't always map to the same definition. The classic (classical) method is that the position number aligns with a fret. So first position = fret 1, 5th position = fret 5. The roman numeral is most often used to indicate the position in modern texts but in some cases I see Arabic numeral notation. When necessary for baring at the position there will be an indication like C1 for a full bar at the first fret for an F chord, similarly C3 for G chord at the their fret. When a partial bar is needed and some open strings in the bass are played you might see a 1/2 in front of the position marking. Position shift are indicated even when no bar is necessary. Classical arrangements usually have them as a courtesy to the musician as there are multiple ways to play a given passage. In most cases I've seen the suggested fingering and position are truly optimal but on occasion not very good. When a few notes need to be played just one fret up from the given position there will not likely be an indication since it seems obvious. I have actually seen modern shredder tab that called the 5th fret the "second position", etc. It is almost as if they divided the neck into large chunks and just labeled the start of each chuck by a new number. I have no idea if this is a modern standard or just the style of that author but I didn't really like it. As for when to know what position to use? If your music does not have specific positions and fingerings for both hands it is not likely arranged by a professional guitarist. In that case you need to do a bit of arranging yourself and this takes time and experience. As an example I arrange bop tunes for myself to play and do not follow the school of thought that I should stay in one position as some do. I tend to look for common phrases, develop an optimal fingering for them then move across the neck, jumping position while keeping the fingering fixed. Again, this takes a lot of time and even professionals will need to go through multiple options before settling down to one.
The guitar is a remarkably flexible instrument for fingerings. In the standard range (E2-B5) some pitches can be played in as many as five different places on the fretboard.
Position itself is a bit of a fuzzy term. It generally means the fret where your index finger will be placed, but in any given position there will be a number of keys where one or more notes aren't under your fingers... and we stretch for those. So the circled part in your question could be played in either first or second position - in first position you'd play the A on the "and" of three with your second finger; in second position that would be a first finger note.
In second position you'd have a choice for where to play the first Bb note: you could reach back to the 1st fret, 5th string, or you could reach forward to the 6th fret, 6th string. (In practice a guitarist will choose the 5th string to avoid playing two consecutive notes with the same finger on different frets).
Position can be marked in several ways:
- With "C" followed by a Roman numeral
- With just a Roman numeral (as in your example)
- With alternative text that means the same thing - I've seen "7th pos." and the like
- Implied through the fingering
In the absence of position marks, here's how I figure out what position to play in:
- I look at the extremes of the range. In your example, that F below the staff MUST be played on the first fret, so I'm going to be low on the fretboard
- Next I look at the key signature. If I put the tonic under the second finger on the sixth string, or under the fourth finger on the fifth string, I will not have to reach for any diatonic notes. Under the fourth finger 6th string I'll reach for one, under the second finger sixth string or first finger fourth string I'll reach for two, under the first finger 5th or 6th string I'll reach for 3.
- I scan for any accidentals, which might take an adjustment (usually a one fret shift either way)
- Finally, if the range of a section extends beyond the position I've chosen, I'll look for the best place to shift. I personally look for any note before the shift is needed that has an open string note, because that buys me a bit of extra time to move my hand.