Early drilling rigs were built on site and left in place. Mobile drilling rigs, introduced in the 1950s (Figure 2-1), are transported by truck and set-up on site and then relocated for the next well. The most visible part of a drilling rig is its derrick, a structure that contains a hoisting arrangement, which includes a crown block and a traveling block. The pulley arrangement is designed to create a large mechanical advantage for hoisting equipment (drill pipe, casing, tubing, blow out control equipment, etc.) in and out of the hole.