Add 313.9 to 12.6. Express the result as a decimal.
We have \[
\begin{array}{@{}c@{}c@{}c@{}c@{}c@{}c}
& & & 1 & \\
& 3 & 1 & 3. & 9 \\
+ & & 1 & 2. & 6
\\ \cline{1-5}
& 3 & 2 & 6. & 5 \\
\end{array}
\]Since $9+6=15$ in the right-most column is greater than 9, we record the 5 and "carry" the 10 to the next column as a 1 (shown over the second 3 in the addend 313.9). The answer is $\boxed{326.5}$.