A polygonal mountaineering chock with at least two sets of opposed faces in which extensions of each set of faces defines an acute angle, and the median distances between the two sets of opposed faces are unequal, whereby the chock can be received in holding position in cracks or openings of two different...http://www.google.com/patents/US3948485?utm_source=gb-gplus-sharePatent US3948485 - Irregular, polygonal mountaineering chock
