# Precedence constraint

### From Glossary

When ordering objects, like jobs to to be performed, this is a constraint that restricts the order: *i must precede j*, denoted
If *order* really means time, and if the model has decision variables and to denote the start times of and resp., this precedence constraint can be written as
where is the time job takes. On the other hand, a precedence constraint need not correspond to real time. For example, could mean *if project is not selected, we cannot select project *. In that case, suppose the model has binary variables and where means project is selected, and means it is not selected. Then, the precedence constraint is represented as: