DCHP-1 Online
keystone (block) 
n. a polygonal block of snow dropped into place in the centre of an igloo dome and serving to lock the structure firmly together.
See also: key-block
- 1864 Then commenced the "spiraling," allowing each tier to fall in, dome-shaped, till the whole was completed, and the key-stone of the dome or arch dropped into its place, the builders being within during the operation.
- 1930 . . . the arch gradually closing over the builder's head, until at length nothing but a square hold is left, which receives the last block--the keystone--cut somewhat wedge-shaped and dropped into its place from without. . . .
- 1958 Finally, the igloo is complete except for one snow block, the keystone block, at its top. . . .
- 1864 Then commenced the "spiraling," allowing each tier to fall in, dome-shaped, till the whole was completed, and the key-stone of the dome or arch dropped into its place, the builders being within during the operation.