Vernier calipers are precise measurement tools which can be used to measure an object's length (such as the outer diameter of a part), an interior measurement (such as the inner diameter of a pipe), and usually a depth below a surface (such as the the depth of a dimple on a casino die). These calipers are composed of three parts: a gradiated main body with a flat 'left hand' protrusion, a sliding collar with a matching 'right hand' protrusion, and an extensible thin metal rod at the end, which moves with the collar. The tool looks something like this:
ID
<----------->

|\         /|
| \       / |                                   depth
| |       | |________                        <--------->
+---+ |_______|_........_\_______________________
|         }}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}     |__________
|     ____________........_______________________|
\   |           |   _____/
\  |           |  /
\ |           | /
\|           |/
<--------->
OD

To measure an object, simply slide the collar back and forth until the part in question is snugly situated in the appropriate measurement location.

The tool has a Vernier scale on it, which permits measurement to fairly high accuracy. These scales are generally somewhat confusing to people who've never seen them before. To read the Vernier scale below, follow the following procedure:

1. Locate the largest centimeter mark not less than the 0 pointer on the bottom (indicated by the carat). This provides the first digit, 2.
2. Locate the largest millimeter mark, similarly not less than the 0 pointer. This provides the second digit, 4.
3. Finally, look along the scale at the bottom to see which of the hashes lines up best with a line immediately above it. In this case it is the '3' hash, which gives the final digit. The calipers are currently measuring a 2.43 centimeter part.

4. _____________________________________________________________
...
...
2cm                                               3cm ...
|                                                 |   ...
|                  2.4   |       (2.43)           |   ...
|    |    |    |    |    |    |    |    |    |    |   ...
----+----+----+----+----+----+----+----+----+----+----+-  ...
^   |   |   |   |   |   |   |  ...
|   |   |   |   |   |   |   |  ...
0   1   2   3   4   5   6   7  ...
...
_____________________________________________________________

=2.43cm

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