Hornbook, was a sheet of vellum or paper used in early times for teaching the rudiments of education, on which were inscribed the alphabet in black or Roman letters, some monosyllables, the Lord's Prayer, and the Roman numerals; this sheet was covered with a slice of transparent horn, and was still in use in George II.'s reign.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Horn Gate * Horrocks, Jeremiah