Page "LR parser" Paragraph 17

Reductions reorganize the most recently parsed things, immediately to the left of the lookahead symbol.

So the list of already-parsed things acts like a stack.

This parse stack grows rightwards.

The base or bottom of the stack is on the left and holds the leftmost, oldest parse fragment.

Every reduction step acts only on the rightmost, newest parse fragments.

( This accumulative parse stack is very unlike the predictive, leftward-growing parse stack used by top-down parsers.