There are two main ways to finger a Stradella bass.
The 2-finger method makes extensive use of fingers 2 and 3 (counting the thumb as 1). Finger 3 typically plays the bass buttons leaving finger 2 for the chords.
The 3-finger method is the one I prefer and relies on finger 4 playing the fundamental bass note, finger 3 playing the chord and finger 2 playing the major 5th bass note.
In both cases you have to use other fingers as well in order to cover all that you have to do.
I like the 3-finger method as I believe that it makes it easier to make up those chords that require two buttons to be held down together (e.g., for a major 6th or major 7th chord) and I also think that it is easier to get a solid "walking bass" rhythm. In addition, I play bass scales starting with finger 4 on the doh or first note of the scale so I don't have to change fingers.
The 3-finger method allows finger 4 to play the counter bass after that finger has been used on the fundamental bass such as when you are moving chordally a 4th, as with a move to the subdominant [say C to F]. You would play finger 4 on the fundamental [in this case the C bass note], then finger 3 on the chord [C major], then finger 4 on the counterbass [E bass note], then finger 3 on the chord [C major] (or perhaps even finger 2 on the 7th [C7]), then finger 4 is available for the subdominant [F].
If you are lucky enough to be able to spread your fingers widely, it is possible to use the three finger method to play the minor and 7th rows with finger 3 while still using 4 for the fundamental bass note and 2 for the 5th bass note. If the stretch is uncomfortable, you may have to play the 7th row with finger 2 but this makes it hard to keep an alternating 1st and 5th bass pattern. If you are moving by a 4th (as described above), using finger 2 to play the 7th chord presents no problem but if you are required to play a 7th chord with repeated fundamental and 5th basses you may have to stretch over with finger 5 to play the bass note of the 5th.
The situation changes for the diminished row because it would be most unusual for the music to call for a fundamental and 5th bass pattern with a diminished chord (not least because the 5th bass note would not be in the diminished chord). This means that you are much more likely to use finger 5 on the bass note and finger 2 on the diminished row - a much more comfortable stretch than trying to use finger 3 for the diminished row. But this is often overtaken by other factors - see "How do I use the diminished chord row?"
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