“Enchiridion” came into English in the 16th century to refer to a portable, hand-sized guidebook. The modern handbook has roots in the enchiridion (related to the Greek word for “hand”), which was traditionally a small, portable manual widely used in ancient Greece and through to the 19th century. An enchiridion was intended to keep useful information — including religious teachings, ethical advice, the rules of poetry, guidance for soldiers, and means of understanding the law — close at hand.