A C block is a block that is shaped like a “C”, so other blocks can fit inside it. These blocks perform the conditions and loops.
C blocks have a “mouth” (it looks like the letter “C”, hence the name) — the blocks that will be played with the C block go in these mouths. All but one of the C blocks consist of one mouth — the other, If () Then, Else, consists of two mouths. When any of the other C blocks are dragged over a stack of blocks — if the C is empty — its mouth extends to wrap around them.
As C blocks are for checks and loops, they are used almost everywhere.