a11y is an abbreviation that stands for the word accessibility — the 11 refers to the number of letters between the a and the y. This makes it a particular kind of abbreviation called a numeronym. That it happens to look like the word ally is a coincidence that likely contributed to its use. The numeronym is most commonly associated with digital accessibility.
I've heard this pronounced like alley, like ally, as a-eleven-y, or as a-one-one-y.
I’ve not yet found concrete evidence that the a11y numeronym was in use before the founding of Twitter, but I wouldn’t rule it out. Regardless, the numeronym was certainly propelled into widespread usage thanks to Twitter’s 140-character limit of old — shaving accessibility’s 13 characters (roughly 9% of 140) down to four characters (roughly 3% of the limit) is certainly enticing.
In long-form prose, I try to avoid abbreviations where possible, including a11y, since I generally hold that abbreviations are more often for the writer’s benefit than for the reader’s. Unless a11y appears in a proper name (such as The A11Y Project, the #a11yTO conference, or Pa11y), I prefer to write out accessibility in full, and generally recommend others do the same.
That said, I still think a11y makes for a useful hashtag on social media — as Eric Bailey points out, searching for accessibility can get you a wide range of results, stretching the meaning as vague as general approachability, whereas searching a11y tends to pull up accessibility practitioners.
External Resources
- a11y is web accessibility, by Eric Bailey
- In Defense of the #a11y Hashtag, by Amanda Rush (archived)
- a11y and a Brief Numeronyms Primer, by The A11Y Project
- a11y = Accessibility, by Adrian Roselli