Relationship Types
FS, SS, FF, SF and lags — where logic quality lives.
What it is
The four relationship types (Finish-to-Start, Start-to-Start, Finish-to-Finish, Start-to-Finish) plus lag/lead, which define how activities are logically tied.
Why it matters
Relationship type and lag decide the driving path. Sloppy logic — excess SS/FF pairs, long lags, leads — produces an unreliable critical path.
In practice
- FS is the default and the cleanest; reach for it first.
- SS and FF are legitimate but heavily overused, which hides the true drivers.
- Lags are durations with no logic or resource behind them — minimise and justify every one.
- A driving relationship is the one actually controlling the successor’s early dates.
Watch for
- Long lags standing in for activities that should exist.
- SS+FF pairs used to force overlap and distort float.
- Negative lags (leads) obscuring the real sequence — avoid them.