Summary simple 3 sided triangle thingie to check that your frame is square on your printer. Yup, can use a box or what ever, but this uses less filament (and time) and, well it looks cooler ;) Little write up on how to check with this and what having an out of square printer can do to your prints.. http://3dprintingexp.blogspot.com/2016/09/square-is-as-important-as-level.html