The parts that turn a router from a freehand tool into a repeatable one. Copying rings are the bulk of it: a ring in the base follows a template, so the cutter reproduces its shape as many times as you need. They come in a spread of diameters, metric and imperial, because the ring you need depends on both the template and the cutter — get the pairing wrong and the copy comes out oversize by a predictable amount.
Alongside them sit centring mandrels for setting the ring true to the spindle, guide rail adapters, parallel fences, extraction hoods and depth stops. Unglamorous, and the difference between a router that cuts where you meant and one that cuts near there.