Made to be used with the Gridfinity system, takes up a 2x1 slot and fits 45 4mm bits. Instead of the normal hexagonal cutouts for the bits this uses 4.5mm holes to make it easier for lower quality prints to fit your bits and allow them to slide in and out easily without excessive sanding or feeling like you are press fitting your bits into your print. With this it is much simpler, if the hole is too tight just ream it out with a 4.5mm drill bit and you're done. Happy Printing!