When aligning a cartridge for placement using an alignment protractor, it is important to remember that you are trying to align the stylus (and therefore the needle) and not the body of the cartridge. There is no guarantee that the stylus (needle) is perfectly aligned within the body of the cartridge, so simply aligning the body of the cartridge may not necessarily produce the desired result. Additionally, many cartridge bodies have non-parallel sides, essentially making it impossible to align the body of the cartridge tangentially with the contact lines of the protractor.