Critical Path Method

Critical Path Method (CPM) charts are similar to PERT charts and are sometimes known as PERT/CPM. In a CPM chart, the critical path is indicated. A critical path consists that set of dependen tasks (each depedent on the preceding one) which together take the longest time to complete. Although it is not normally done, a CPM chart can define multiple, equally critical paths. Tasks which fall on the critical path should be noted in some way, so that they may be given special attention. One way is to draw critical path tasks with a double line instead of a single line.
Tasks which fall on the critical path should receive special attention by both the project manager and the personnel assigned to them. The critical path for any given method may shift as the project progresses; this can happen when tasks are completed either behind or ahead of schedule, causing other tasks which may still be onschedule to fall on the new critical path.

Reference:
A Professional's Guide to Systems Analysis", Martin E. Modell, 2nd. Ed. McGraw Hill, 1996.
http://studentweb.tulane.edu/~mtruill/dev-pert.html
Accessed: September 11, 2008