This is an adapter for the PLAyPHAb spool. This will allow the spool to fit correctly on the Ultimaker spool holder. Good for the Ultimaker printer family. It may work for other printers, but I don't know what size spools they use. You need to print them in pairs. I printed this with 0.15 layer height and 100% infill. I think you can get away with less infill though.