I’m often asked what the difference is between a trio with two violins and a cello and one with a viola instead of the second violin.
The simplest explanation is that the viola has a lower tone. The second violin has the ability to reach higher notes when needed while the viola can go lower.
The viola’s sound is sometimes described as mournful. . . but in a good way. It doesn’t stand out the way the cello does but instead adds a wonderful depth to the middle voice.
Which pairing of instruments should you choose? In many cases the choice is dictated by the music that you’ve selected; some pieces are written specifically for two violins and cello. If you’re still not sure, leave it up to your musicians and don’t worry about it. Most people can’t tell the difference.