A plywood square, sometimes called a drywall square, is a large square used to make straight lines on large sheet material like plywood or drywall. A chalk line or even a straight 2x4 can also be used for this purpose, but this tool is faster.
Like the carpenter's square, I prefer a plywood square that is made from aluminum like the Empire 48 in. Drywall T-Square. It’s lightweight and yet durable, and won't rust if left outside.