Frankie's neck bolts are called hex bolts. Why hex? Well, because the head (the top) of the bolt has six sides and is hexagon shaped*.
Hex means six in Greek. In robotics, a hexapod is something with six feet. In music, a hexachord is a series of six notes.
Back to Frankie: Bolts come with hundreds of different kinds of heads but hex is probably the most common type (good news for Frankie if she ever gets a screw loose). People like hex heads because they can be turned with lots of different tools and because they're one of the easiest to turn in small spaces.
If you really want to sound like you know what you're talking about, call a bolt a kind of fastener (y'know, because it fastens things together . . . like necks) which is a big category of hardware including screws and nails.
Frankie's neck bolts are all threaded, but her shoes have a smooth shank for extra style. You are going to look like such a pro next time you need to fix something!