Use the equation to find the length of the missing leg measure.

Let's solve the equation step by step using the provided format:

1. Evaluate the powers:
\[
a^{2} + 16^{2} = 20^{2}
\]
Calculating the squares, we have:
\[
a^{2} + 256 = 400
\]

2. Isolate the variable:
\[
a^{2} = 400 - 256
\]
\[
a^{2} = 144
\]

3. Take the square root of both sides:
\[
\sqrt{a^{2}} = \sqrt{144}
\]

4. The missing side length is:
\[
a = 12
\]

So, the missing side length is \( a = 12 \).