Three gear types with true involute geometry: classic spur gears, ring gears (the internal-tooth donut at the heart of planetary drives), and straight racks that turn rotation into linear motion. The one number that matters is the module — tooth size in mm — and it must match across every part that meshes. Set it directly, or switch the sizing mode to circular pitch (module × π) if you're matching an existing gear train measured tooth-to-tooth. Tooth count sets the ratio (and a rack's length — teeth × pitch). Backlash adds running play between mating teeth: 0.2–0.3mm keeps printed pairs from binding. Clearance (0.4mm default) drops each tooth tip away from its mate's valley floor so layer seams and dust have somewhere to go. Ring and rack backing is the solid material behind the teeth — 2mm prints sturdy; go thicker if you'll screw through it. A hex bore on a spur captures a nut so you don't need a set screw. Print tip: all three types print flat with no supports; for working drives use 3–4 perimeter walls — walls carry tooth load, infill doesn't. Involute math from Leemon Baird's public-domain gear generator; the gear-type and sizing controls follow Jason Koolman's Gear Generator (CC BY 4.0) — thanks, Jason.