Tape measures are great, but the when you're doing inside measurement, like in a window, you have to awkwardly bend the tape to get your measurement and that bend makes it inaccurate. It turns out that most tape measure cases are a measure themselves.
On the back of most cases, you'll see the width of the case listed. This means you can use the case to measure to an inside corner, then add the length of the case to the number you get. People who measure things a lot probably already know this, but if not, it's a pretty handy bit of knowledge.