A single under is one rope pass under your feet per jump. A double under is two rope passes in the same jump, before you land.
Keep the same tall shape, quiet feet and wrists turning the rope. A double needs more airtime and faster rope turns. Two single jumps are not one double under: you land between singles; both passes of a double happen before that landing.
What stays the same, and what changes
Compare
Single under
Double under
Passes per jump
One
Two before landing
Airtime
Small bounce
More time for the second pass
Rope speed
One wrist turn
Two faster wrist turns
Shape
Tall body, quiet feet, wrists turning; arms stay close
Single under 1 rope pass per jumpDouble under 2 rope passes per jump
Quarter-speed demonstration. Timing is illustrative, not a target cadence. Watch for the double's second pass before the feet land. Both figures start paused; Play moves both together.
I can do singles; what next?
Try these existing drills where they fit your practice. Each has its own dose; this is no mandatory workout.