The groove is rectangular in cross section. And is cut with a rectangular tool. Imagine cutting an o-ring groove in the face of a part. This groove is rectangular in cross section yet has curved sides. The grooves I need to cut are exactly the same. Curved sides. I drew up a screw with the groove sides parallel to the screw head top. But it has disadvantages. Not as much squeeze on the o-ring unless the groove is shallow. Which then raises the possibility of the o-ring extruding over the edge of the groove and getting damaged. The customer didn't like it. I didn't either except it would have saved the customer some money. By putting the groove normal, that is to say perpendicular, to the angled face, then the groove can be cut to the proper o-ring dimensions. In fact, people keep telling me to look up in a catalog or online at the o-ring specs to get the groove dimensions correct. This is the first thing I did. This is one of the reasons why the groove is normal to the angled face. BTW, thanks for mentioning me, albiet obliquely, in the same sentence as feynman. Eric