# Happened-Before Relation

Most of the time it doesn't matter if all the processes share the same time, but they **need to agree on the order in which events have occurred**. The concept of order is necessary.

### Happened-Before Relation

* If **a** and **b** are two events of the same process, and **a** is before **b**, then **a -> b**.
* If **a** is the **sender** of a message, and **b** is the **receiver** of the same message, then **a -> b**.
* If **a -> b** and **b -> c** the **a -> c**.

This partial order of events is also the concept used in systems with concurrent processes.
