What is "main couple"

Main \Main\ (m[=a]n), a. [From Main strength, possibly influenced by OF. maine, magne, great, L. magnus. Cf. Magnate.]

1. Very or extremely strong. [Obs.]

   That current with main fury ran.
   --Daniel.

2. Vast; huge. [Obs.] ``The main abyss.''
   --Milton.

3. Unqualified; absolute; entire; sheer. [Obs.] ``It's a man untruth.''
   --Sir W. Scott.

4. Principal; chief; first in size, rank, importance, etc.; as, the main reason to go; the main proponent.

   Our main interest is to be happy as we can.
   --Tillotson.

5. Important; necessary. [Obs.] That which thou aright Believest so main to our success, I bring. --Milton. By main force, by mere force or sheer force; by violent effort; as, to subdue insurrection by main force. That Maine which by main force Warwick did win. --Shak. By main strength, by sheer strength; as, to lift a heavy weight by main strength. Main beam (Steam Engine), working beam. Main boom (Naut.), the boom which extends the foot of the mainsail in a fore and aft vessel. Main brace.

   1. (Mech.) The brace which resists the chief strain. Cf. Counter brace.

   2. (Naut.) The brace attached to the main yard.

      Main center (Steam Engine), a shaft upon which a working beam or side lever swings.

      Main chance. See under Chance.

      Main couple (Arch.), the principal truss in a roof.

      Main deck (Naut.), the deck next below the spar deck; the principal deck.

      Main keel (Naut.), the principal or true keel of a vessel, as distinguished from the false keel.

      Syn: Principal; chief; leading; cardinal; capital.