4 ms·To make sure your 90 is really a 90. Easy and quick to check.by pdbwimsey 5y agoTo make sure your 90 is really a 90. Easy and quick to check.hervature 5y agoYou can do that with the square itself.thechao 5y agoYou can check the corner is square with a square. Checking the walls and the room can be done with a string.