Another type of special-purpose pencil is used for marking multiple-choice answer sheets and similar purposes. A scanner detects the mark locations optically. Nowadays, any number 2 or HB pencil usually suffices, but scanners were less sensitive in the past and special lead formulations were required. These pencils are referred to by terms such as Test Scoring, Mark Sensing, Electrographic, Machine Scoring, and OMR (Optical Mark Recognition).
Many are black, suggestive of their dark marks:
The pair of Detect-O-Graphs at the bottom are another example of a connection between the Linton and Venus companies.
Here are some more, including the IBM Electrographic (see Pencil Talk):
Some modern Mark Sheet pencils from Japan:
The Tombow Mono one came with a plastic point protector:
And finally, a mechanical variety by Dur-O-Lite: