When You make it to the top of a climb you often have to repel back down to where you left all your gear. This is accomplished by building a anchor at the top of the wall that can be used to repel down safely.

The anchor consists of two anchor points with some style of webbing or rope connecting them down the the climber. However as the angle tween the climber and each of the anchor points increases the force on the cord and anchor goes up and it is more likely to fail.

As seen in the picture above as the angle between the anchor point increases the force they feel goes up even for the same climber mass. This is because the anchor must support the climber in the Y direction so the force on the anchor is F / cos(angle). This means that as the angle between the anchor points is small, the force on the anchor is close to the force from the climber. As the angle increases cos(angle) becomes smaller so F on the anchor gets larger and are more likely to fail.

Because of this climbers try and keep the angle between their anchor points as small as possible as shown in this sliding-x anchor below.