between the lines

Probably from glosses and commentaries which were written between the lines and provided the reader with additional information not explicitly stated in the text.

between the lines

1. By implication or way of inference; indirectly.
   • 1881, “On Reading between the Lines”, in London Society, volume XXXIX, London: William Clowes and Sons, page 531:

     The lines give us the letters, but we have to look ‘between the lines’ for the spirit and the intention.

   • 1978 April 9, Joseph Lelyveld, “China Watch”, in The New York Times‎^[1], New York, N.Y.: The New York Times Company, →ISSN, →OCLC, archived from the original on 20 December 2025:

     Chinese have to live this critical assessment. If they read about it at all, it's between the lines.

   • 1981 December 5, Rudy Grillo, “Gay Moments in Straight Music”, in Gay Community News, volume 9, number 20, page 9:

     Much of gay history over the years and centuries has been written between the lines. Few openly gay publications existed before the 1970's and the grapevine was "juicy" indeed. Jargon, gestures, oft-changing codewords, dress, color, and other devices have always been employed by gays and lesbians in order to make their presence known to their kin.

   • 2020 May 19, Sean Gentille, Josh Yohe, and Rob Rossi, quoting Mike Emrick, “Doc Emrick on empty arenas, Mike Lange and his first ‘event of gravity’”, in The Athletic‎^[2], New York, N.Y.: The New York Times Company, →ISSN, →OCLC, archived from the original on 20 December 2025:

     You would always have a sense, wouldn’t you, of what was at least being said between the lines and have a good idea as to how long the pause was going to be.

• “between the lines, phrase”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.