A relationship defines how an activity relates to the start or finish of another activity or assignment. An activity can have as many relationships as necessary to model the work that must be done. These relationships are used together with activity durations to determine schedule dates. Relationships can also exist between activities in different projects; this type of relationship is referred to as an external relationship. You can enter comments about a relationship, for example to explain the need for the relationship or how it is configured.
Activities that are dependent on one another are known as predecessors and successors, where the first activity is the predecessor and the second is the successor. Between these two types of activities, there are four possible relationship types:
Finish to Start: The successor activity cannot start until its predecessor finishes.
Finish to Finish: The successor activity cannot finish until its predecessor finishes.
Start to Start: The successor activity cannot start until its predecessor starts.
Start to Finish: The successor activity cannot finish until its predecessor starts.
A permitted modification to these logical relationships is called lag. Lag values can be positive numbers (a delay, slower, deceleration of progress) or negative numbers (lead time, faster, acceleration of progress). For example, in a Finish to Start relationship, if you specify a ten-day lag (+10), the successor activity cannot start until ten days after the predecessor has finished. Likewise, if you specify a negative ten-day lag (–10), the successor activity could start ten days before the related predecessor activity finishes.
Note: Lag entered in a unit other than hours is converted to hours based on the predecessor activity's calendar.