Question

What is a primary difference in tonal quality between a fretted instrument (e.g., viola da braccio or guitar) and a fretless one (violin or oud)? Why is there a difference?

Answer 1

The main difference, other than the lack of frets, is tone. Fretless instruments get a much warmer sound that emphasizes the lows; mostly because the string is touching skin which is much softer than the metal fretwire. Fretted instruments get a more trebly sound with a sharper attack. The instrument the electric bass was meant to replace has no frets so people consider it to be the "true" sound of the bass. It's mainly up to personal preference, though.

Always buy a fretted bass if you are a beginner unless you can already play the double bass.