an apparatus and its method of use with a conventional tape measure confirms right angles of the corners of a rectangular structure . the apparatus is a two - part tool including a tape measure tab part and a tape measure tape part . the tab part secures a tape measure tab at one corner of the rectangular structure as the tape measure tape is pulled from the tape measure housing diagonally across the structure . the tape part of the apparatus is secured to the diagonally opposite corner of the structure and is used to check the diagonal distance measurement of the tape across the structure . the process is then again performed between the other diagonally opposite corners of the structure to obtain a diagonal distance measurement across the other corners to determine if the two diagonal measurements match and confirm the right angles of the rectangular structure .