English uses a number of suffixes to denote nouns. I like to call them nounifiers. They transform an adjective or verb into a noun. Common nounifiers include:

-ment: government, supplement, commitment

-tion: indication, anticipation, implementation

-ity: density, propensity, university

-ness: vagueness, quietness, compactness

-ship: fellowship, sportsmanship, friendship

–hood: brotherhood, neighborhood, parenthood

-age: storage, stoppage, passage

–ance: governance, temperance, tolerance

Two other common nounifying suffixes are -ence and -ency. What I find interesting is that while some words are formed with one (e.g., evidence, turbulence, opulence) and some with the other (e.g., frequency, tendency, solvency), there are others that can be formed either way. You could say “It’s a matter of expedience.” or “It’s a matter of expediency.”

There’s no right version. They are equally correct options, though there are subtle and barely discernible differences in meaning. That doesn’t mean, however, that one isn’t preferred over the other in usage. A quick look at Google’s N-gram viewer (which provides comparative data on word use across a large body of published works going back to 1800), shows that expediency is more common than expedience, though the gap has narrowed and both words have become less frequent over time:

The trend looks almost identical for despondence vs. despondency:

Coherence, on the other hand, has grown more preferred to coherency over time:

And some words reversed in preference at some point. Competence surpassed competency around 1890:

And belligerence passed belligerency as recently as 1980:

An interesting situation occurs with emergence and emergency. Both words are common, but have developed distinctly different definitions over time. There are differences between insurgence and insurgency as well according to Merriam-Webster, but those words are often used interchangeably. Not so with emergence and emergency.

As this type of word arises in your writing, I suggest you take a moment to look up each option and select the one that best represents your meaning rather than simply choosing the one that seems most familiar.

Here’s an incomplete list of words that use both -ence and -ency suffixes:

• belligerence, belligerency
• coherence, coherency
• consistence, consistency
• competence, competency
• complacence, complacency
• despondence, despondency
• emergence, emergency
• expedience, expediency
• insurgence, insurgency
• lenience, leniency
• persistence, persistency

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