Even though the word discernible means “able to be seen or understood,” it ends in –ible, not –able.
Two other synonyms of discernible are perceptible and visible, which also end in –ible.
The adverb forms of these adjectives end in –ibly: discernibly, perceptibly, and visibly.
The noun forms of these words end in –ibility: discernibility, perceptibility, and visibility.
However, three other synonyms of discernible end in –able:
observable, detectable, and noticeable.
Why is this? Especially when we are talking about an ability!
Well, it’s just another vagary of the crazy English language.
If you Google them, “discernible” has 4 million hits and “discernable” has 2.7 million hits, so either a lot of people are misspelling it, or the latter is becoming more accepted. Some dictionaries say the latter is “rarely” used. They must be old dictionaries!
But the truly educated ones, including the members of the Peanut Gallery, know to use “discernible,” because they are very discerning individuals.
Profound Quote of the Day:
“Our educational system is like an automobile which has strong rear lights, brightly illuminating the past. But looking forward things are barely discernible.”
– Hermann Oberth, German scientist, 1894-1989