When is an arc an arch? Is an arc ever in fact an arch?
An arc is first of all a curve that is made up of part of the circumference of a circle. So it has a particular form which may well be like that of an arch, but it’s always part of that circumference in this case. It can also be the electrical discharge that jumps from one point to another (so lightning forms an arc: not in this case a nice tidy bit out of a circumference) and finally we have the metaphorical use in a “story arc” in a fiction book, film, TV series or play (often across several episodes of a TV series) which traces the development of a plot or side plot. The verb to arc means to move with a curving trajectory, which could include arching over something.
An arch is a physical thing rather than a mathematical concept or a plot device (though you can have over-arching ideas that act as a sort of umbrella across a narrative or other story). So it’s a symmetrical curved (though that curve can be quite pointy) structure that supports a bridge, a wall, etc. It’s also the inner side of the foot, which is the same thing but in nature rather than constructed. The verb means to form an arch.
So an arc has a specific shape unless it doesn’t, and an arch is a physical thing unless it’s a metaphor. But you don’t have a story arch and most arches couldn’t be said to form part of a circumference of a circle.
You can find more troublesome pairs here, and here’s the index to them all!
LibroEditing proofreading, editing, transcription, localisation 
Satyendra Bhandari
February 6, 2019 at 3:58 pm
A curving line, not always fitting exactly a part of the circle, is also an arc, in my view. Whereas in case of an arch, the symmetry aspect of a circular arc is,essential to equally distribute the load
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Liz Dexter
February 7, 2019 at 9:15 am
In my understanding, an arc would be symmetrical so as to always form part of a circle. E.g. a rainbow forms an arc because you could make it into a hypothetical circle. An arch might be part of a circle but it can be pointed, part way between, etc., in architectural terms.
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Don Massenzio
February 7, 2019 at 12:24 pm
Reblogged this on Author Don Massenzio and commented:
Check out this post from the Libro Editing blog that helps us decide between using arc or arch?
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Henrietta Watson
February 14, 2019 at 1:37 pm
Reblogged this on All About Writing and more.
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Nathan Kindle
August 28, 2022 at 4:11 am
I’ve been noticing this issue a lot lately. Read a lot of books on the Kindle app, especially in the litrpg genre, and almost all the authors keep using the wrong word. They’ll say things like, “the arrow arched over the field” or something along those lines. It seems a lot of authors on Kindle get arc and arch mixed up. I guess its what happens without a decent editor.
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Liz Dexter
September 12, 2022 at 6:01 pm
It’s a difficult one for people and yes, not having an editor takes one protective line out for picking up these things.
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