The double crunch is a bodyweight exercise focused on the muscles of the core, the rectus abdominis or "six-pack" muscles in particular. It's a very simple exercise that involves "crunching" the knees and elbows toward each other using the abdominal muscles, while keeping the back flat against the ground.
Targets both upper and lower ab muscles
Simple exercise that can be done by all fitness levels
Works well with other ab-focused movements in a circuit