What is Leads and Lags?
A lead accelerates a successor activity — it starts before its predecessor finishes (shown as a negative value, e.g., FS-3 days). A lag delays it — a forced wait after the predecessor finishes (FS+5 days). Neither adds work; they adjust the timing relationship between activities.
The classic lag is physical: concrete needs curing time — nothing to do, just wait. The classic lead is overlap: start landscaping when the building is 80% done. Mixing these up is an easy exam point to lose, so anchor them: lead = head start, lag = wait.
Worked example
Foundation pour → 7-day lag → framing: the crew isn't slow, the concrete is curing. Meanwhile in the same schedule, "write user documentation" carries a 5-day lead on "feature freeze" — the writers start while developers finish, compressing the timeline without adding risk to either task.