Definition of keystones:
(n) :
(architecture) The top stone of an arch.
(n) :
Something on which other things depend for support.
(n) :
A native or resident of the American state of Pennsylvania.
(n) :
(retail) A retail price that is double the cost price; a markup of 100%.
(n) :
(baseball) The combination of the shortstop and second baseman.
(v) :
(transitive) To distort (an image) by projecting it onto a surface at an angle, which for example causes a square to look like a trapezoid.