One of the mysteries of the English language finally explained.
1Army. A formation of regularly spaced men, horses, etc.; especially one in which individual soldiers are two or more yards apart. Usually with preceding in (also †at).
2Navy. With in. A formation in which individual ships are spaced at regular intervals, especially at four cables apart. Now rare.
Early 17th century; earliest use found in Gervase Markham (?1568–1637), author. From open + order.
open order/ˌəʊp(ə)n ˈɔːdə/
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