The typical Martin guitar nut shown here on a left-handed D-18. Martin angles the base with two parallel vertical sides and a large top radius. This can give the impression that the nut needs to "dovetail" in from the side (like an old Fender fret) but actually it drops straight down into the slot. I've stop doing these Martin nuts from complete scratch because StewMac sells a nice oversize un-slotted, natural bone blank with the correct angle and radius. These strings may look (be cut) a bit "deep" but they were actually slotted at the factory. The distance between strings is equal and the setup and height relative to the frets is good.