c diagonal is a speed equal to the speed of light, the fastest possible speed at which any effect can propagate.
A fuse formed by a diagonal row of cells will burn at the speed of light if not stabilized. Methods of stabilization include using pre-blocks (as in canoe and snake), tails (as in cis-fuse with two tails, and nines. Many such stabilized diagonal fuses based on the diagonal line exist, such as such as baker and beacon maker.
