A point in left-linear grammars. In (i:.j), j is the non-terminal storage point, i==0 always for the non-terminal, while i>=0 for terminals, which are on the right of the non-terminal. (This is why left-linear grammars are called left-linear: they recurse on the left).

PS: all this left/right talk deals with the RHS of a production rule, the LHS is always a non-terminal ;-)

A point in right-linear grammars.