The "split verb rule" says that an adverb must not be placed between an auxiliary and the following verb. On this account, you should never write "you should never write", but rather "you never should write". In an earlier post, I followed (what I thought was) the lead of James Lindgren ("Fear of Writing", California Law Review 78(6):1677-1702, 1990) in attributing this bizarre idea to The Texas Law Review Manual on Style. But in a comment this evening, Jon Weinberg cited Allen Black, "Judge Wisdom, the Great Teacher and Careful Writer", 109 Yale L.J. 1267 (1999-2000):
He was death on split infinitives and split verbs. A sentence such as "The burdened vessel was slowly proceeding down river at the time of the collision" would never survive.
Since John Minor Wisdom would have learned his attitudes towards such things in the 1920s, and the Texas Law Review's Manual does not seem to have appeared until the 1950s, Jon suggested that we need to look elsewhere for the source of this peculiar prejudice. And indeed, a quick Google Books search turns up a more promising source – Henry Watson Fowler and Francis George Fowler, The King's English, 1908, section 46 on "'Split' Auxiliaries":
Some writers, holding that there is the same objection to split compound verbs as to split infinitives, prefer to place any adverb or qualifying phrase not between the auxiliary and the other component, but before both.
The Fowler brothers don't specify who "some writers" are, and Google Books turns up no earlier hits for "split auxiliaries" or "split verbs". Perhaps some readers can track this one down further.
In any case, the Fowler brothers reject the rejection of "split auxiliaries", instead making some interesting remarks about interaction with information structure and stress:
Provided that the adverb is then separated from the auxiliary, no harm is done : 'Evidently he was mistaken' is often as good as 'He was evidently mistaken', and suits all requirements of accentuation. But the placing of the adverb immediately before or after the auxiliary depends, according to established usage, upon the relative importance of the two components. When the main accent is to fall upon the second component, the normal place of the adverb is between the two; it is only when the same verb is repeated with a change in the tense or mood of the auxiliary, that the adverb should come first. 'He evidently was deceived' implies, or should imply, that the verb deceived has been used before, and that the point of the sentence depends upon the emphatic auxiliary ; accordingly we should write 'The possibility of his being deceived had never occurred to me ; but he evidently was deceived', but 'I relied implicitly on his knowledge of the facts ; but he was evidently deceived'.
They then make free to decide that Burke, Beaconsfield, Ruskin, Charlotte Bronte, and the London Times all often did it wrong — because for the Fowlers, placing an adverb between an auxiliary and the following verbal material is not only not forbidden, it's actually obligatory, except in certain circumstances:
In our first two examples below the adverb is rightly placed first to secure the emphasis on the auxiliary : in all the others the above principle of accentuation is violated. The same order of words is required by the copula with whatever kind of complement.
I recognize this truth, and always have recognized it.
Refined policy ever has been the parent of confusion, and ever will be so, as long as the world endures. — Burke.
They never are suffered to succeed in their opposition. — Burke.
She had received the homage of . . . and occasionally had deigned to breathe forth . . .— Beaconsfield.
He ordered breakfast as calmly as if he never had left his home.— Beaconsfield.
Miss Becky, whose sympathetic powers never had been called into action before. — Ferrier.
They now were bent on taking the work into their own hands.— Morley.
There may have been a time when a king was a god, but he now is pretty much on a level with his subjects. — Jowett.
They both are contradicted by all positive evidence. — W. H. Mallock.
Religious art at once complete and sincere never yet has existed. — Ruskin.
Not mere empty ideas, but what were once realities, and that I long have thought decayed. — C. Bronte.
So that he might assist at a Bible class, from which he never had been absent .— Beaconsfield.
If we would write an essay, we necessarily must have something to say. — Bygott & Jones.
The protectionists lately have been affirming that the autumn session will be devoted to railway questions. — Times.
Visitors no longer can drive in open carriages along the littoral. — Times.
It still is the fact that his mind . . . was essentially the mind of a poet. — Times.
To whom in any case its style would have not appealed. — Times.
To go wrong with not is an achievement possible only with triple compounds, where the principal division is of course between the finite (would) and the infinitive with participle (have appealed). ' Would not have appealed ' must be written, though at an enormous sacrifice of 'distinction'.
This enhanced value of old English silver may be due partly to the increase in the number of collectors ; but it also has been largely influenced by the publication . . . — Times.
Mr. Fry showed to a very great extent his power of defence . . . To-day, if runs are to be of importance, he very likely will show his powers of hitting.— Times.
This is a canonical case of a self-appointed authority inventing a grammatical theory, observing that elite writers routinely violate the theory, and concluding not that the theory is wrong or incomplete, but that the writers are in error. Still, we're not talking about the grammar of English, but about the history of mistaken ideas about the grammar of English. And in this case, the Fowlers' argument is against a "split verb rule" rather than for it.
So this leaves us with two possibilities. Perhaps the "split verb rule" started with the Fowlers' "some writers", whoever they were, and spread through the legal profession despite the Fowlers' best efforts to quash it. Alternatively, a generation of spectacularly careless readers of The King's English understood as a "rule" a pattern that the Fowlers explicitly rejected as a violation of the "principle of accentuation".
Update — Compare Austin Osgood Hubbard, Elements of English Grammar (1827), whose Rule 16th, Obs. 2 asserts that
Those adverbs which qualify compound verbs, are usually placed after the auxiliary.
Or the section "Of the syntax of adverbs" in A Complete English Grammar, Compiled from Louth, Johnson, L. Murray, Cobbett, &c., with Additions, Corrections, and Improvements (1836), which begins
Adverbs, though they have no government of case, tense, &c., require an appropriate situation in the sentence, viz., for the most part before adjectives, after verbs active or neuter, and frequently between the auxiliary and the verb; as, "He made a very sensible discourse ;" "He spoke unaffectedly and forcibly, and was attentively heard by the whole assembly."
Or Brandon Turner, A New English Grammar (1840), which observes that
For the placing of adverbs, no definite general rule can be given. Those which relate to adjectives, immediately precede them; and those which belong to compound verbs, are commonly place after the first auxiliary.
Or many other sources from the first half of the 19th century, which uniformly and straightforwardly assume that the common practice, then and now, is also the correct practice. It would be interesting to understand why and how this sensible discourse was transmuted into the absurd "split verb rule", accepted a century later as gospel by someone as intelligent as John Minor Wisdom seems to have been.
MWDEU notes that
Copperud 1970, 1980 talks about an erroneous idea widespread among newspaper journalists that adverbs should not separate auxiliaries from their main verbs (as in "you can easily see" or "they must be heartily congratulated"). This bugaboo, commentators agree, seems to have sprung from fear of the dread split infinitive. Copperud cites five commentators on the subject, all of whom see no harm in placing an adverb between the parts of a verb, and one of whom (Fowler 1965) prescribes such placement. [...] Since dividing the auxiliary from the verb with an adverb has been approved at least since Lindley Murray 1795, it would seem that Fowler is justified in calling the avoidance a superstition.
In his various works on English usage, Bryan Garner agrees, calling the split verb rule "nonsense" and citing various authorities in favor of normal adverb placement. But the only clue that he gives us about the source of the nonsense is an indirect allusion to the Texas Law Review Manual of Style:
A fairly well-known manual on legal style long cautioned its readers to avoid splitting verb phrases with adverbs …
So perhaps the split verb rule was purely a folk superstition, elevated to a point of elite principle in the early 20th century by some now-forgotten law professor in Texas or Louisiana. But it seems unlikely that the Fowler brothers' "some writers" came from the U.S. gulf coast. The history remains a mystery.