GuidesDependency types
Task dependency types: finish-to-start, start-to-start, finish-to-finish and start-to-finish
Updated
A dependency says that one task's timing is tied to another's. The task that constrains is the predecessor; the task that is constrained is the successor. There are four ways to tie them, named for which end of each task is linked.
The four types
| Type | Code | Meaning | Gantt arrow |
|---|---|---|---|
| Finish-Start | FS | Successor starts after predecessor ends | Solid |
| Start-Start | SS | Successor starts after predecessor starts | Dashed |
| Finish-Finish | FF | Successor ends after predecessor ends | Dashed |
| Start-Finish | SF | Successor ends after predecessor starts | Dashed |
Finish-to-start (FS)
The successor can't start until the predecessor finishes. This is the default and by far the most common link: you can't paint a wall until it is plastered, and you can't test a feature until it is built. If you are unsure which type to use, it is almost always this one.
Start-to-start (SS)
The successor can't start until the predecessor starts. Use it for work that runs in parallel but must not get ahead: laying pipe can start once trenching has started, and proofreading can start once writing has started. It usually comes with a little lag, so the second task has something to work on.
Finish-to-finish (FF)
The successor can't finish until the predecessor finishes. Use it when two tasks overlap but one can only wrap up after the other: the final inspection can't finish until the last trade does, and documentation can't be finished until the feature it documents is.
Start-to-finish (SF)
The successor can't finish until the predecessor starts. It is rare, and most plans never need it. The classic case is a hand-over: the old system can't be switched off until the new one has started running, and a night shift can't end until the day shift has started.
Lag and lead
Any link can carry an offset. Positive lag adds a gap — concrete must cure for seven days before the formwork is stripped, so that link is FS with 7 days of lag. Negative lag, called lead, lets the successor overlap — drywall can start two days before the electrical rough-in is fully finished, so FS with 2 days of lead.
Lag is a deliberate gap with a reason behind it. It is worth keeping that record intact rather than folding it into dates, so that when the predecessor moves, the gap moves with it.
How Waterfall PM writes them
In Waterfall PM's Predecessors column a link is the predecessor's outline number, the type's code and any lag — the same notation Microsoft Project uses. 1.2FS+3d is finish-to-start from task 1.2 with three days of lag; 4SS is start-to-start from task 4; 7FF-1d is finish-to-finish from task 7 with a day of lead. Finish-to-start arrows are drawn solid on the Gantt chart and the other three dashed.
A plain drag of a bar never rewrites a link's lag — it pins the task instead. Only an Alt+drag retimes the driving link, on purpose.