Vernier calipers are used to measure items with great accuracy. Normal measurement units are in centimeters, down to the millimeter engraved along the long arm called the fixed main scale.
A vernier caliper is initially zeroed out to obtain an accurate positive measurement, but a negative reading is a special application of the vernier caliper to measure the leftover thickness of a work item that has a portion missing, such as a hole.
When zeroing out the tool, it's possible to receive inaccurate positive or negative readings if the fixed scale does not line up perfectly with the movable scale for reading. Care must be taken to keep the tool free from damage that can cause a misalignment of the two scales.
A vernier caliper can measure depth, the outside and inside of an object, and a little known measurement called step. Step measurement can measure the distance from one step, or level, to another on a work item.