The four relationship types

A relationship ties one end of an activity to one end of another. There are two ends per activity, start and finish, so there are four combinations.

Type	Read it as	Example from the lab
FS finish-to-start	B can start once A has finished	Pressure test starts after backfill is finished
SS start-to-start	B can start once A has started	Pipe laying starts 2 days after excavation starts
FF finish-to-finish	B can finish once A has finished	Pipe laying finishes 1 day after excavation finishes
SF start-to-finish	B can finish once A has started	The temporary bypass can stop once the pressure test starts

The first activity named is always the predecessor. Most relationships in a healthy schedule are finish-to-start: they are the easiest to read and the hardest to misuse.

How each type moves the successor

The forward pass works out the earliest start (ES) and earliest finish (EF) of each activity. For a successor B with duration D and a lag L:

Type	Rule	So B’s ES is at least
FS	B’s start ≥ A’s finish + L	EF(A) + L
SS	B’s start ≥ A’s start + L	ES(A) + L
FF	B’s finish ≥ A’s finish + L	EF(A) + L − D
SF	B’s finish ≥ A’s start + L	ES(A) + L − D

FF and SF tie down the successor’s finish, so the lab subtracts the duration to find the earliest start that still respects them. When an activity has several relationships, each one gives a candidate, and the forward pass takes the largest.

The SS + FF pair

Overlapping crews are common: excavation moves along the route and the pipe crew follows two days behind. An SS+2 says exactly that. But on its own it lets the pipe crew finish whenever it likes, even before the trench is dug. Adding an FF+1 closes that gap: the last length of pipe goes in one day after the last length of trench is finished.

Work it through for Lay pipe (6 days), behind Excavate trench (ES 3, EF 11):

• The SS+2 gives ES ≥ 3 + 2 = 5.
• The FF+1 gives EF ≥ 11 + 1 = 12, so ES ≥ 12 − 6 = 6.
• ES is the larger, 6: pipe laying runs Days 7 to 12, and here the FF drives.

Primavera P6 accepts both an SS and an FF between the same two activities. Microsoft Project allows only one link between two tasks, so there you would model the finish dependency differently.

Start-to-finish: rare but real

SF reads backwards and is easy to get wrong, which is why many scheduling standards discourage it. The classic legitimate use is a handover: the old thing has to keep running until the new thing starts. In the lab, Run temporary bypass is the successor of Pressure test with an SF. The test starts at the beginning of Day 15, so the bypass must run until the end of Day 14 at the earliest, and with 5 days of work it runs Days 10 to 14.

Your turn

In the lab, use the CPM math tab to see which relationship drives Lay pipe. Then delete the FF and watch pipe laying finish on the same day as the trench, before putting it back. Finally, make excavation longer and watch everything downstream move, the bypass included.