When finger picking open chords one is supposed to do Thumb - bass string (root string), Index - third string, Ring - second string etc.
The bass string used should be the "root" of the note. e.g. If playing an E chord, one should pluck the E string, if playing a D, then the D string.
My question is:
If one is using barre chords to, say, play an A# chord (based on E shape on E string), then which bass string should be plucked? The A or the E string?
Barre chords are transpositions of open chords; the root note stays on the same string:
Any open chord that you can transpose as a barre chord will follow the same rule, e.g. a C chord has the root note on the 5th string (third fret), so a C-shape barre chord also has the root on the 5th string (third fret above the barre).
This is of course also true for the scale degree of the other notes in the chord; which strings play the root, third, fifth, ... in an open chord doesn't change when you play that shape as a barre chord:
Q: What bass string to use?
A: Use the string that has the note you want to play as the lowest note. If you want to play the chord's root note as the lowest note, then use the string that has the root note. If you want to play some other note as the bass note, then use whatever string has that note.
The bass note and the root note of the chord are not necessarily the same thing. The bass note is the lowest played note, and if it's not the root, then the chord can be said to be an "inversion" - or it can be an alternating bass line, which doesn't really sound like an inversion, when the non-root note is played on a weaker beat.
If you learn to play alternating bass, then you'll have to learn which of the notes in a chord fingering is the root note. In a common alternating bass pattern, you first play the root as the bass note, then the 5th as the bass note, and repeat. Root, 5th, root, 5th, ...
Here is Dm and A7 played with open position chords:
Here are the same chords with barre grips:
And here's what it sounds like:
If you want to play "fingerstyle" guitar, then the alternating root - 5th bass pattern is a basic thing you need to know. After you get that going fluently, you can start using more advanced bass lines.
In addition to Your Uncle Bob's very clear answer.
There isn't really 'you are supposed to...'. The bass note is often the root, and yes, played with thumb. Main reasons - thumb is just where it needs to be, and the sound is good for a bass note.
Fingers can and do go where necessary. That's not always the case then of index here, ring there etc. A lot of the finger patterns I use vary considerably. There's one which uses thumb on 3rd then 4th, and a couple of fingers on the top two strings. So nothing is written in stone. I quite often use pinky - others rarely do. Sometimes (often) you make your own 'rules'!
With finger picking, there's sometimes an option to play an alternating bass pattern - 1-5-1-5. For that, using E shapes, 6 and 5 are used. For A shape, the reverse (with the barre obviously on all 6 strings!). That works also for C shape. G shape is tricky, with 1 and 5 being strings 6 and 4 - not too good. And D shape works somewhat with 4 and 5, again with an extended barre.
