(From Fun With Knots Book, p. 40)
This is used for lashing together parallel spars and for forming "sheer legs" which support bridges and the like.
Begin with a Clove Hitch around one of the spars. Take seven or eight turns about both spars. These turns need not be very tight, but the frapping turns taken around them should be pulled very snug. Finish with a Clove Hitch on the spar opposite to that on which the first clove hitch was laid.