A set of timbers consisting of a cap and a single post. If the ground is loose and must be supported over the side or back, lagging, commonly of 2-in (5.1-cm) boards, is used. These boards extend from the center line of the post or cap to the middle of the next post or cap. If they are placed touching each other, such an arrangement is called tight lagging; if a few inches apart (which depends on the nature of the ground to be held back), it is called open lagging.