This word is acceptable for play in the UK dictionary that is being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. interlinear

adj. Contained between lines; written or inserted between lines already written or printed; containing interlineations

Between lines; interlinear.

adj. written between lines of text

"Within these lines she made little dots at the top and bottom of stubby perpendicular strokes, and strange interlineal hieroglyphics, and sweeping curves, all of which would have puzzled an"

interlinear