Length of Concatenation

Theorem
Let $S$ and $T$ be words, and let $ST$ be their concatenation.

Then:


 * $\left\vert{ST}\right\vert = \left\vert{S}\right\vert + \left\vert{T}\right\vert$

where $\left\vert{S}\right\vert$ denotes the length of $S$.