(Small Swiss teacher)
FIG method in the preamble, there are four kinds of preamble and dependency relationships, as follows (the former: Representative preamble Quest:: Representative successor task): 
(picture from the network)
Some people interpret this relationship as "front led to the latter," In fact, this is not accurate. I believe that following this statement is the most accurate and most understand:
FS (finish-to-start): previous activities completed, back to the beginning.
SS (start-to-start): previous activities began, back to the beginning.
FF (finish-to-finish): previous activities completed, back to complete.
SF (start-to-finish): previous activities began, back to complete.
Understanding these differences and relationships are very important. For example, in the renovation project must finish carpentry job to dry painters, which is typical of (former) completed - (after) the start (FS type).
Some work has between closely related, such as power supply work not yet begun, welders can not begin to work, which can be thought of (before) the start - (after) the start (SS type).
In the site, if the security officer's job security check is not completed, even if the other work, "finished the job" can not be considered complete, it can be understood as (former) completed - (after) completed (FF type).
SF is a special present. From the front and rear timing point of view, since it is the "front of work" than work behind the first start it is natural, therefore, under normal circumstances, as a condition for the start in front of the back to complete, appears to be not a necessary condition. However, if there is a time difference between the constraint condition, the situation is different, for example, in some cases, there may be a special operating constraints:
**** After the three-day event began nine days 1, which lasted four days Activity 2 to complete, so in this case, the activity need 2 days to complete it?
Come to think about!