Types of Micrometers:
Outside micrometer, (aka micrometer caliper), typically used to measure wires, spheres, shafts and blocks
Inside micrometer, used to measure the diameter of holes
Depth micrometer, measures depths of slots and steps.

This is a Starret T436XRL-9 Outside Micrometer from Amazon

This is a Metric "Caliper" style micrometer made by “Draper.” It’s manufactured from hardened stainless steel. Slider displacements are amplified by a rack and pinion mechanism and indicated on the dial. Four way measurement: internal, external, depth and step. Reading 0.02mm. Slider has lockscrew function. Display packed in plastic storage case.