A part of the rock/wall with a sharp change in angle. In bouldering, this is most often a vertical feature of about 90 degrees in angle - like the corner of a house. From the French for "edge". Here's a classic arête problem at Stanage in the Peak District (see also working a problem!)
A corner is, obviously enough, a feature like the corner of a room where you often have the chance to bridge between walls with your hands or feet (or both) to make progress. It's the inverse of an arête. Johnny Dawes' classic climb The Quarryman is a masterclass in corner climbing, and in making holds and moves exist where, to mere mortals, they don't. And in the power of lycra. In this video James Pearson repeats this classic on the Welsh slate.
A slab is a section of rock which is just off vertical (angled slightly away from the climber), lending itself to delicate, balance-y problems often requiring great technique and footwork. Here's a classic slab problem on the superb red circuit at Bas Cuvier in Fontainebleau, which is a lot harder than he makes look.
Overhangs are the bread and butter of the burly bouldering beast. Often requiring great finger, core, and upper body strength, these are steep climbs where it's often impossible to keep your feet underneath you (though it's usually a great idea to keep your feet on the wall wherever possible). A pair of more aggressive climbing shoes, designed to channel power through your toes and enable them to "pull" on small features in the rock, may help on this type of problem. Speaking of burly bouldering beasts, here's Josh crushing a classic overhang in Portland, Dorset.
Crack climbing follows fissures in the rock, which can be used as climbing holds through a variety of inventive (and often painful) finger, foot, hand, elbow, knee, chin and other bodily contortions. Certain types of rock (e.g. gritstone, found in the Peak District) lend themselves to this type of route due to the abundance of this kind of natural feature. Tom Randall and Pete Whittaker are the UK's leading crack climbers, and get up to all sorts of unlikely looking things. Indoors, crack climbs are rare but we do have a crack volume feature that comes out every now and then...
Volumes are hollow features (often triangular) which can be moved around the bouldering wall in order to change the angles of the walls. They are often used in bouldering competitions to create weird angles and shapes for competitors - there are some great examples near the start of this video. At Boulder Brighton the light green volumes are "house" volumes, meaning that they can be used as part of any coloured climb. Other volumes in different colours should only be used as part of the circuit that they match in colour. (Disclaimer: there is currently one yellow volume at the centre also being used as a "house" volume. This is Gaz Parry's fault for being way too quick off the mark when he came to set for us earlier in the year, it was up and set before we could even blink.)
Now you know your way around the rock (or wall), get over to part 4 - different hold types.
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