There are two types of micrometers in common use. One is the screw gauge, which uses a retractable spindle with precisely machined screw threads to obtain an accurate measurement. The other is the caliper micrometer, which uses sliding caliper arms to obtain measurements. For increased accuracy, both types will typically use a Vernier scale in addition to the regular fixed scale. The accuracy of the micrometer is given by the lowest increment on the Vernier scale, so a 0.02mm micrometer will have a Vernier scale marked in 0.02mm increments.
Take a measurement with your micrometer. Different micrometers, such as inside, outside or depth, have different procedures for taking measurements. Use your micrometer's locking mechanism, which is usually a thumb lever or a thumb wheel, to hold the measurement in place.
Read the number on the fixed scale that the "0" on the moving Vernier scale is pointing to. Usually, this line will be pointing in between two numbers. If this is the case, read the lower of the two.
Read the value from the Vernier scale that perfectly aligns with any value on the fixed scale. Only one of the lines on the Vernier scale will align with the fixed scale at any given time.
Combine the two readings. In the photo, the micrometer has an accuracy of 0.02mm. The fixed scale reading is 3.0mm because the "0" on the Vernier scale is pointing between the "3" and the "4." The Vernier scale reading is 0.58mm, because that is where the Vernier scale aligns with fixed scale values. The combined reading is 3.58mm.