Defining milestones

Apparently my brain can remember the gist quite well while forgetting the source. Before I forget again:

For picking the milestones there is only one relevant rule. Milestones must be concrete, specific, measurable events, defined with knife-edge sharpness. Coding, for a counterexample, is "90 percent finished" for half of the total coding time. Debugging is "99 percent complete" most of the time. "Planning complete" is an event one can proclaim almost at will.

Concrete milestones on the other hand, are 100-percent events. "Specifications signed by architects and implementers," "source coding 100 percent complete, keypunched, entered into disk library", "debugged version passes all test cases". These concrete milestones demark the vague phases of planning, coding, debugging. (Fred Brooks - The Mythical Man-Month: Essays on Software Engineering)