Working through a piece of code by hand using pen and paper, representing variables, arrays and pointers by boxes, grids and lines, etc. Very common in the days when computing power was expensive or when access to a computer was limited. Immensely valuable at teaching how programming works, practically essential for understanding algorithms but almost totally pointless for testing GUIs. Seems to be a forgotten art.

''Is this another name for DeskChecking?'' I guess so, although desk checking may also refer to ensuring that your colleagues haven't left you yet another (ha ha) booby trap.

Still quite useful for debugging certain state-based or network-related problems. Sometimes a debugger just can't handle too many interleaved paths.