Don, from the TF Glossary
KEY. Small element, usually wedge-shaped, used to lock
a joint or, if a shear key, to prevent sliding of one member
over another.
KEYED BEAM. Two or more beams laminated together
with crossgrain keys let in between the beams to prevent
slippage during bending.
SHEAR BLOCK. Wood block dapped (q.v.) partially to
adjoining parallel laminae in a built-up chord, designed to
resist shear between the two members or to transfer load
around a discontinuity such as a scarf, and properly oriented
parallel to the grain, so that shear block end grain bears
upon chord end grain.
SHEAR KEY. Wood block oriented perpendicular to
(across) the grain. Easier to assemble, and can be tightened
if wedge-shaped, but not so resistant to compression
as a shear block (q.v.).
In my usage a key has a flat surface oriented square to the force, except as a sloping shear block. The example of the rotated key is just a simple visual example of how sloping surfaces can impart force, a peg has a sloping surface that varies from parallel to square to parallel from the direction of force.
So, how does riving force matter? In a quiet frame not so much, the pegs function as place holders however in a tension generating frame with open joinery. such as a frame using free tenons, I do have concerns over the chance that the tenon will split. So in that case, square pegs or keys seen like a better choice.
