Johnson Level & Tool 48 in. Drywall/Plywood T-Square
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$47.27 at the time of publishing
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/drywall square that is made from aluminum like the Johnson Level & Tool 48 in. Drywall T-Square. It’s lightweight and yet durable, and won't rust if left outside.