What is a Summary Activity (Hammock)?
A summary (hammock) activity is a single bar representing a group of related activities — its dates and duration derived entirely from the detail beneath it. Summaries exist for communication: executives read a 20-bar summary schedule; planners maintain the 2,000-activity detail that feeds it.
The discipline: summaries are computed, never planned. The moment someone drags a summary bar to "fix" a date without touching the detail, the schedule has two owners and zero truth.
Worked example
The owner's monthly report shows one bar, "Marine Works — Apr 12 to Nov 3," hammocked over 240 detailed activities of dredging, piling, and quay construction. When three piling activities slip, the summary bar stretches automatically and the November date moves — visibly, honestly, and without anyone editing the report by hand. The summary's job is to be a window, not a painting.