Many handsaw handles are designed to have two (sometimes more) angles to the spot where they connect to the blade, with one running perpendicular to the saw. As shown in the video from The Maker below, you can use those angles to your advantage by lining one of them up along the edge of the piece of wood you plan to cut.
With the handle lined up, you can then use a pencil along the flat (back) of the saw blade to trace your cut lines. And by using both angles in conjunction, you can set yourself up for either 90- or 45-degree cuts. Perfect for fitting around or inside of most corners.
Bear in mind that this trick won’t work with every single type of handsaw. Most modern tooth saws and backsaws should have the necessary handle shape to accommodate tracing out your cutting angles. However, older tools and saws with incompatible grip sizes or shapes (like hacksaws or jigsaws) likely won’t cut it.
