Summary This is just a blatant copy/remake of an awesome little tool! Just a tad taller! Original item - (red one in the images) https://www.thingiverse.com/thing:280974 The original was great but for one thing, after I added a glass bed it ended up being shorter than the end stop for the Z axis. SO I made a slightly taller one. what it does- It checks the alignment of the X axis rods to the rods in the Y axis to make sure they are square/level to each other. Use before leveling the bed to get a MUCH better and more accurate leveling. Clip on the Y rods and lower BY HAND the Z down till the X rod just touches, go to other rod on Y axis and adjust.. never do this with power, you can severely damage the printer by bending the rods if you use the motors!!.