Keying
Core concepts
Keying covers how keys are cut, measured, and made to operate a lock. A candidate must understand the parts of a key (bow, shoulder, blade, tip, bitting), the difference between a key cut by code versus one duplicated by tracing, and the concepts of spacing (distance between cuts) and depth (how deep each cut is). Know how to read a bitting chart, how to use calipers or a key gauge to measure depths, and how to identify a keyway by its profile. Understand top versus bottom pinning conventions and the role of the key's shoulder as the reference point against the cylinder face.
Traps to watch
A common trap is confusing spacing with depth: spacing is horizontal position along the blade, depth is vertical cut height. Another is assuming a duplicated key is as accurate as a code-cut key; tracing copies wear and error. Questions may test that the shoulder, not the tip, is the reference for insertion depth on most keys. Also watch for keys that appear identical but differ by one depth increment, which changes the operating pin height.
Rule of thumb
Measure with a gauge or calipers rather than eyeballing; one depth increment off is enough to prevent operation. When in doubt, cut by code from the manufacturer's bitting rather than duplicating a worn original.
Worked example
A customer brings a worn key that sticks. Measuring the original shows a cut one increment deeper than the factory bitting because of wear. Cutting a new key to the factory code restores proper operation, whereas duplicating the worn key would carry the error forward.