The process of writing between the lines of an instrument; that which is written between the lines of a document.
An interlineation frequently appears in a contract that has been typed and signed. If the parties agree that a sentence is to be inserted between the lines to clarify a particular provision, the new sentence is known as an interlineation. The new line should be initialed and dated to indicate that both parties are aware of and agree to its insertion. An interlineation results in the alteration of an instrument.