Rounding the corners of things. Imagine a perfect cube. The edges will be very, very sharp, and this is bad for any number of obvious reasons. To fillet the ed

Rounding the corners of things. Imagine a perfect cube. The edges will be very, very sharp, and this is bad for any number of obvious reasons. To fillet the edges is to round them off: now the cube is much easier to handle. These are "outside" fillets, and you could perhaps make them with a router bit. There are also "inside" fillets (say you "drilled" a perfectly cylindrical hole partway into the cube; an inside fillet would be along the circular bottom edge of the hole), which can't be made after the fact (to do so would involve adding material). The geometry only gets more complex from there: most things aren't cubes or cylinders! Fillets are at once extremely simple, extremely complicated, and extremely important. They are siblings to chamfers, which are the same idea but instead of being round just involve breaking the edge. You could chamfer the edge on that hypothetical cube with a single touch from a sander or (big) file. Fillets require more care.