Fit a smaller seatpost into a bigger frame. The shim grips the post, the slit closes when you tighten the collar, and a top lip keeps it from dropping into the frame.

Stack spacers — drop straight onto the steerer.

Bottom-bracket / crank spindle spacers — slide on with clearance.

Shims for the bar. Stem = the clamp in the middle (skinny bar, big stem). Lever / accessory = the grip zone — put a standard 22.2 mm bell or lever on a narrow 19 mm kids bar, or a road lever where it doesn't belong.