*wel-, *ule-

The core idea of this root was:

turning, revolving, winding

The most notable branch is the one around the Latin verb volvere (to roll) which is the origin of words like these:

• involve (“roll in”)
• evolve, evolution (“roll out”)
• revolve (“roll in return”)
• revolt, revolution (“roll in return”)
• devolve (“roll backward”)
• convoluted (“rolled together”)
• volume (originally: a roll of a manuscript”)
• vulva (“the wrapper”)

Also part of the family are willow, wallow, whelk, waltz and well (as in a “water well”) and probably also to walk, which originally may have been about rolling objects around to move them.

Oh and from the Greek side we have helix and helicopter.