ABSTRACT
8.11. It has been said above (4.31f.) that in the middle English period and in the beginning of the modern period that was the usual relative word, but that its sphere of use has since then been narrowed through the encroachments of the wh-pronouns, especially in the literary language. Nowadays that is used very seldom indeed in non-restrictive clauses, and even in restrictive clauses there is a strong tendency to use who and not that in reference to persons, and a less strong tendency to use which in reference to things.
