In English a great many nouns from Latin offer you a choice of -ce or -cy. Some people cry foul at this. Why does English clutter the mind with needless variant pairs like complacence / complacency?
In an orderly tongue all such pairs would behave as civilly as absorbent (n.) / absorbency. "That which absorbs" / "quality of absorbing." There’s a Latin crispness to it; they're precise as numbers.
Thank God we haven't got one. Both those words, absorbent, absorbency, arose in the 18th century. They smell like it; a scientist’s couple of words, not a poet’s. Poets choose words they can chew.
Properly, the -cy, from its Latin and Greek origins, indicates "quality or state of being ___." The -ce forms ought to mean "a fact or instance of ____." The absorb group shows this nicely.
English, however, plays by the old rules only when it chooses. In these sorts of nouns, we tend to make our shorter words end in -cy, longer ones in -ce, regardless of sense. The ear of English keeps proportion.
And many words in -ce take -ies plurals (irrelevancies). And when there is a differentiation in meaning, we tend to make the -cy form the concrete word (dependency, contingency, emergency).
This all makes for murky going in the grammar. Where does excellence stop and excellency begin? What makes an ascendance not an ascendancy? If you stop too long to puzzle it, doubt creeps up you like quicksand. Conveniency? Dependance? Is benevolency a word? Your assurancy sags.
IN CASE of EMERGENCY
Emergence-emergency is a visible instance of a -ce/-cy doublet put to good use.
The two nouns together cover, roughly, three senses: "a rising above water of what had been submerged," used of dead bodies or continental shelves, is the sense closest to the etymology of the Latin verb.
The second meaning is perhaps directly from emerge: "the process of issuing from concealment," as the sun does in an eclipse.
The third is "sudden appearance, unexpected occurrence" (of some condition, etc.). When concrete, this is the ordinary modern use of emergency. The other two now go with emergence.
The sense-split seems logical: emergence, as the apparition of Venus shedding a veil of evening cloud, and emergency, as in "the jute mill is exploded!"
But this is not original, and the divergence was not firm until recent times. Both emergence and emergency are attested in English from the early or mid-1600s. Dictionaries until the late 19c. consistently treated them as the same word, with all senses assigned to both forms. (Almost all dictionaries did likewise with all the -ce/-cy pairs.)
The words emergence and emergency were a single entry in Johnson's dictionary (1756) and its reissue under Todd (1818).
Ash's dictionary (1775) has separate entries, identifying emergence as "the act of rising from any surrounding element, the act of rising into view;" and also "a sudden occasion, a pressing necessity." For emergency it says merely "not so common a Word," and defines it only as "an emergence."
[OED's editors scolded Ash elsewhere for multiplying his entries to bulk up his book, but in doing so here Ash noted a fact the OED found worth citing and found nowhere else.]
Emergence and emergency also formed a single entry in the U.S. in Webster (1832). They continued to be a single entry, with all senses under it, in popular dictionaries through the 1870s. OED (1989) has prose citations for emergence in the meaning of our emergency into mid-19c., from the likes of Sir Walter Scott and Jane Carlyle.
In 1883 the massive Imperial Dictionary gives emergence and emergency separate headings and entries. It defines emergence only as a rising out of fluid or an act of issuing from.
The entry for emergency has the definition "sudden occurrence" in it, but the Imperial gives its 1. sense as "Same as emergence (which see)."
The Encyclopaedic Dictionary (1887) finally has two entries, with the two senses fully divided: both meanings are under each word, but they are marked as active or "obsolete" according to the modern division of them.
By 1926, Henry Fowler can write in his guide to "Modern English Usage" that "The two are now completely differentiated."
EMERGENCY MEN
The modern notion of an "emergency" focuses on its disruptiveness more than its "coming out from within" a sea of routine life. Our emergencies don't emerge, they explode. Or threaten to. Sirens scream and everyone runs around and eventually someone gets hurt from that, but as often as not whatever set it off is a false alarm.
Tracking the words, the 1860s jump out as the time emergency begins to feel like the modern word.
In the American Civil War, to be precise. "Emergency" was used frequently in the North in reference to rebel invasions, real or imagined. The Lincoln administration and the governors had to be careful about words; "war" and "rebellion" had constitutional consequences.
By the end of the war emergency was well-established, and regarded cynically, at least in the Pennsylvania valleys most susceptible to rebel raids:
[Newville Star, Aug. 13, 1864; "fail" = typo for "tail"]
The word emergency doesn't shift a gear in 1860 and suddenly define differently. The public awareness of the thing-itself evolved beneath the word. In daily life, in widespread use, across the North, the word begins to describe events that start to resemble the modern sense of emergency: an immediate crisis with bells clanging and people dropping everything. Etymologically, this is not a "from," not an emergence. But it certainly is part of the story of the word.
It is not unusual for such -ce/-cy noun pairs to sit undifferentiated for centuries, then split senses. Redundance and redundancy have been around since c. 1600, but in mid-20th century redundancies acquired a euphemistic sense in regard to job layoffs.
The same pattern holds with residence / residency, both from Middle English. The hospital sense of the latter dates only from the 1920s. Frequence and frequency go back to the 16th century; the physics sense that split them only to the 1830s.
The GUILT of INNOCENCY
If emergency is essential, innocency ("state or quality of being innocent") is not.
Unless you're a poet. Here's James Russell Lowell:
Here Love in pristine innocency bold
Speaks what our grosser conscience makes a crime.
Where innocence would break his iambic and his pentameter. But that is its only virtue. There is no difference in meaning. The -y is there only to fill out Lowell's metrical line.
He has company. Shakespeare's plays seem to use innocence much more often, but innocency is in them, and often when the verse demands the syllable.
If truth and upright innocency fail me
[Henry IV, Part 2]
Shakespeare's plays were printed without his supervision. There are obvious errors in the texts that suggest unobvious ones. There's an innocency in "King John" where innocence clearly would be the better metrical fit. Alexander Pope in his great 1720s edition of Shakespeare changed this to innocence.
In "Richard II," there's an innocence that the scrupulous Capell (1768) changed to innocency, and this was accepted by later editors as "very likely" what the playwright meant. Meanwhile in "Richard III," "God and our innocency" may be a misprint for "innocence," which is found there in the 1st quarto.
Altering Shakespeare is the brain surgery of literary editing. Yet all this debate about Shakespeare's innocences and innocencies was based solely on prosody, on the metrics of the iambic line. No sense difference between the two words was mentioned or implied.
And that shines a light on the answer to the question, why does English have these variant pairs? You can scrape Latin and propose two streams of transmission over centuries for innocence and innocency. But, before 1850 or so (and back to 1530 or so), part of the answer will always be, "poetics."
They're poetic switch-hitters, these word. Ambidexters. The old dictionaries weren't wrong; they are not two words, they're a single word with a suppleness of form that makes them suitable for metrical speech in syllabic feet. For Shakespeare as for Swinburne (who also used innocency), and everyone between.
Including writers of prose. Innocency is in Ascham, Tyndale ("In the innocency of myne hert and with clene hondes have I done this") and other writers of cadenced prose in early Modern English. They also were the early users of emergency (Thomas Browne, Donne in the sermons). Such writers lean toward the longer form for its voluptuous majesty.
There is good reason for what seems to us an inanity.
Say they are superfluous doppelgangers, clumsy clones made by versifiers for their convenience and no more. Useful only for the drumbeat of a dead poetics.
Yet those monsters many times have slumbered centuries in verse, then stood up as useful citizens of the dictionary.