The segment addition postulate is a fairly obvious-seeming postulate which states that:

If a point $B$ lies on a line segment $\overline{AC}$, then $AB+BC=AC$.

That is, the distance from one endpoint to the other is the sum of the distances from the middle point to either endpoint.

Example:

Find the length $XY$ in the figure shown.

By the Segment Addition Postulate, $XY+YZ=XZ$.

Substitute the given values.

$XY+11=17$

Solve for $XY$.

$XY=6$