Usage Shibboleth

The writers of grammar books were preoccupied with certain contested constructions, such as the split infinitive, multiple negatives, the use of who/whom, etc. In some cases their judgments were based on a direct appeal to the equivalent usages in Latin, but in others they seemed to be arbitrary (and copied from one another). The links below take you to two grammars, in which you can read what they say about the distribution of "will" and "shall" among the first person, second person, and third person forms. No one seems to know where John Wallis derived the rule, but it does not seem to come from the usage of his contemporaries in the seventeenth century. His "rule" was widely copied, and one sometimes continues to encounter it today, although it's clearly a rearguard action as shall retreats into obsolescence.

John Wallis, Grammatica Linguae Anglicanae. Oxford, 1652. Even if you can't read Latin, you can make out the distribution from his examples at the bottom of p. 94 and top of p. 95. 

Lindley Murray, English Grammar, adapted to the different classes of learners. York, 1798; new edition, 1823. Go to p. 88 for shall and will; then browse around to other sections. Murray is something of an outlier when it comes to shall and will.