(wrong coordination of an independent sentence with a defining relative clause).
It is not easy to see why the relative more than other words should be mishandled in this way; few would write (but see p.61, s.f.) 'This league we kept and has proved advantageous'.
The condensed antecedent-relative 'what' is only an apparent exception to this universal rule. In the sentence 'What I hold is mine', 'what' is only object to 'hold', not subject to 'is'; the subject to 'is' is the whole noun-clause 'what I hold'. Sentences of this type, so far from being exceptions, often give a double illustration of the rule, and leave a double possibility of error. For just as a single 'what' cannot stand in different relations to two coordinate verbs in its clause, so a single noun-clause cannot stand in different relations to two coordinate main verbs. We can say 'What I have and hold', where 'what' is object to both verbs, and 'what is mine and has been fairly earned by me', where it is subject to both; but we cannot say 'what I have and has been fairly earned by me'. Similarly, we can say 'What I have is mine and shall remain mine', where the noun-clause 'what I have' is subject to both verbs, and 'What I have I mean to keep, and will surrender to no man', where it is object to both; but not 'What I have is mine, and I will surrender to no m.an'. Of the various ways of avoiding this error (subordination, adaptation of verbs, insertion of a pronoun, relative or otherwise), that chosen by Miss Bronte below is perhaps the least convenient. Her sentence is, however, correct; that from the Spectator is not.
Not mere empty ideas, but what were once realities, and that I long have thought decayed.—C. Brontë.
Whatever we possessed in 1867 the British Empire possesses now, and is part of the Dominion of Canada.—Spectator.
'Things that were once realities, and that I long have thought decayed'; a pair of defining clauses.
The condensed 'what' must of course be distinguished from the 'what' of indirect questions, which is not relative
