Closest Order is just that - and it's very close. Unsuitable for musketeers, this distance is appropriate for pikemen about to thrust themselves into push of pike.
William Barriffe does not include Closest Order in his discussion of distances. Closest Order, however, is in "The A.B.C. of Armes," 1616, by 'J. T. Gent' (most likely a Gentleman with the initials J. T.) and "Souldiers Accidence," 1635 (2nd ed. corrected & edited by G. Markham), and both provide the distance as shoulder to shoulder.
Closest Order, as used in today's re-enactments by the Sealed Knot
in England, is
a very tight formation, likely tighter than what was usually done in
the 17th century (that is, until two units of pikemen
actually came together, which often resulted in a writhing mass of
struggling flesh). All in ranks and files press
shoulder to shoulder, and then pack together even more.
In Closest Order, the many pikemen of a unit become a single block
of brawn and determination. As two
blocks of re-enactment pikemen collide, they cease to act for any
audience and a genuine contest ensues. When "Closest Order!" is shouted, the pulse of all participants