Blue pencil (editing)

A blue pencil is a pencil traditionally used by an editor or sub-editor to show corrections to a written copy.

The colour is used specifically because it will not show in some lithographic or photographic reproduction processes; these are known as non-photo blue pencils. For similar reasons, sometimes red pencils are used since their pigment will not reproduce by xerography.

With the introduction of electronic editing using word processors or desktop publishing, literal blue pencils are seen more rarely, but still exist in metaphor.

The "blue pencil test" is used by courts of law as a method for deciding whether contractual obligations can be partially enforced.

Blue pencil is also used pejoratively to mean censorship.

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Wikipedia