Another interesting one, with one common usage and one not-so-common, but still the ability to be confused. And they actually have fairly opposite meanings, so you don’t want to be going around confusing them!
To prescribe is, of course, firstly to issue a medical prescription – “the doctor prescribed me a new sleeping pill to reduce my nightmares about grammar mistakes”, but the definition that causes confusion is the one that means “recommend with authority”. I think it’s a bit stronger than that, actually. For example, in my English Language studies, we often talked about reference books being descriptive (describing what language was doing, including changes, including those that people might not like – new uses or coinings, the dreaded text-speak, etc.*) or prescriptive (describing what language SHOULD be doing and laying down the law about it (like I do in these posts … but more so). So that implies a slightly stronger meaning than simply recommending with authority.
Having gone on about prescribing, the other one is fairly simple. To proscribe is to condemn or forbid. So your Mum might proscribe the wearing of mini-skirts, or a company might proscribe inter-colleague relationships.
One letter, opposite meanings!
* of course, the dreaded text-speak didn’t actually exist yet back in the Dark Ages when I was a student. But you get the idea. It was probably L33T Speak** that people were getting upset about)
** ask a techie who’s around 40.
You can find more troublesome pairs here.
LibroEditing proofreading, editing, transcription, localisation 
Nick
September 16, 2011 at 9:23 pm
I was asked by one of my students today what the difference between des- words and dis- words was – eg describe and disappointed. I said dis- is a negative prefix, but I’m not sure of the root for des- . Still, I was impressed by the question!
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Liz at Libro
September 17, 2011 at 6:05 am
Interesting, although I fear they are barking up the wrong tree, slightly. The prefix here is de- not des- and if you check that in the OED, which I admit I just did (someone was shocked the other day that I don’t know all grammar and word use out of my head: I do know a lot of it, but even the stuff I’m *sure* of I still check if I’m publishing it to the world!) you get …
prefix
1. (forming verbs and their derivatives) down; away: descend, deduct – completely: denude
2. (added to verbs and their derivatives) denoting removal or reversal: de-ice
3. denoting formation from: deverbal
origin: from Latin de ‘off, from’; sense 2 via Old French des- from Latin dis-
(that last bit is interesting)
Checking the etymology of describe, I found it’s the “down” meaning there – de / down scribe / write, because the older meaning is to mark out or draw a geometrical figure (think of ice skaters describing a perfect figure of eight)
Hope that helps!
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