A finish-to-start link means the successor cannot start until the predecessor finishes; start-to-start and finish-to-finish model overlapping work. Start-to-finish reverses the normal flow and GAO says its use is widely discouraged. Every activity's start should be driven by a predecessor and its finish should drive a successor.
The four relationships
GAO's Schedule Assessment Guide describes three working forms. Finish-to-start (F–S): the successor cannot start until the predecessor finishes. Start-to-start (S–S): the successor cannot start until the predecessor starts. Finish-to-finish (F–F): the successor cannot finish until the predecessor finishes. None of them forces the two activities to happen at the same moment.
Start-to-finish (S–F) directs a successor not to finish until its predecessor starts. GAO calls it a theoretical fourth combination that reverses the expected flow of logic. Cases used to justify it can usually be rewritten as F–S logic by splitting activities or finding better F–S predecessors.
An example of F–F: a test report may be drafted while testing is under way but cannot be completed until testing is complete.
Why most links should be finish-to-start
GAO says most relationships in a detailed schedule should be F–S. They are intuitive, easy to trace and show clearly which work must finish before other work begins. S–S and F–F are valid for overlapping work, but too many of them in a detailed schedule may point to an optimistic schedule or modelling shortcuts, can hide the critical path, and tend to create dangling logic.
Dangling, missing and redundant logic
Every activity should have at least one predecessor and one successor. The exceptions are the start milestone, the finish milestone, and justified inputs or outputs such as a hand-off to an external partner.
Dangling logic is an improper tie to an activity's start or finish. GAO gives two checks. Any activity with an F–F predecessor should also have at least one F–S or S–S predecessor, or nothing drives its start. Any activity with an S–S successor should also have at least one F–S or F–F successor, or its finish drives nothing.
Networks should also be free of circular logic and of redundant logic. If A, B and C are already linked F–S in a chain, an extra F–S link from A to C is unnecessary.
Where the exam catches people
Believing start-to-finish is a normal choice for overlapping work: GAO says its use is widely discouraged.
Believing S–S means both activities start on the same day: it only means the successor cannot start before the predecessor starts.
Believing an F–F predecessor alone is enough: the activity also needs an F–S or S–S predecessor to drive its start.
Believing extra links make the network safer: redundant links should be removed.
Believing heavy use of S–S and F–F shows a mature schedule: in a detailed schedule it may signal optimism or shortcuts.