A climbing safety harness doesn't just hold you off the ground, it should, if properly constructed and fit to your body, help keep you upright in the event of a fall. Some climbing harnesses also have gear loops, places for you to clip extra climbing gear and carry it as you progress upward.
Most climbing harnesses used in North America fit around the waist and legs only. In Europe, climbers using a chest harness that fits around the shoulders and ribs as an additional aid to keep you upright in the event of a fall is very common.
All climbing harnesses have the same basic parts: A waist loop that goes around your waist, one leg loop that goes around each upper thigh and at least one hard point at the front of the harness, which is designed as the appropriate place to tie in your climbing rope or attach your belay device.
As of 2009, some contemporary harness models use quick buckles that don't require doubling back. These can never be completely unfastened. But many harnesses still use D- or O-shaped buckles, and it's not enough just to thread the appropriate strap through the buckle. It must be doubled back through the buckle to keep it from slipping loose.
Once you've put your harness on and buckled it, test to see if it's tight enough by sliding a flat hand between the harness and your body. It's OK if there's enough space to insert your hand, but if you have room to turn your hand around, the harness is too loose.
Climbing safety harnesses come in different sizes--referred to by numbers or letters--depending on the manufacturer. Some are also created specifically to fit for women climbers.