What are the four types of activity dependencies?

The four dependency types are finish-to-start, start-to-start, finish-to-finish, and start-to-finish. Finish-to-start is the most common: the successor cannot start until the predecessor finishes, such as pouring concrete after building forms. Start-to-start means both activities start at the same time, such as writing code and writing unit tests simultaneously. Finish-to-finish means both activities must finish together, such as editing and proofreading ending at the same time.