# MPI Communicators
**Communicators** are groups of processes in MPI. Messages sent on one communicator cannot be received on another, even between the same ranks with the same tag. `MPI_COMM_WORLD` is the default, including all processes launched by `mpirun`. Within a communicator, each process has a **rank** (0 to N-1) and the group has a **size**.

`MPI_Comm_split` partitions an existing communicator into sub-communicators. Each process passes a `color` to determine which group it joins and a `key` to control rank ordering within the new group. Processes with the same color end up in the same new communicator. This is useful for creating communicators that span a single node, a single row of a process grid, or any other subset.

```c
int color = rank % 2;  // even and odd ranks form separate communicators
MPI_Comm half;
MPI_Comm_split(MPI_COMM_WORLD, color, rank, &half);
// use half...
MPI_Comm_free(&half);
```