Multiply and divide

I’ve been always wondering, when speaking and you have to say 2 times less, I mean:
We use: double to express something multiplied by 2
We use:triple … multiplied by 3
But how do you express when a thing is divided by 2?
Should I use half?
What when it’s divided by 3?
One third?

The verb form of "to cut into half" is halve, so you could say, e.g.:

The estate was halved.

if it was split into two equal (or roughly equal) parts.

There may be a verb for "divided into thirds," but if there is, I can't think of it, and I don't think it's a common one. I'd probably say:

The pizza was divided into thirds.

• Please unaccept my answer and wait a day or so. There might be a verb for "cut into thirds" and I may be unaware of it. It's considered good practice to wait at least half a day to give others a crack at the question, and you're less likely to get more helpful answers if you accept an answer too soon. I've even seen a few cases where an answer was blatently wrong but the O.P. accepted it before others had a chance to review it and comment on it – that's even worse! I don't think I've said anything wrong here, but I'd much rather give others time to verify that. English varies around the globe.
– J.R.
Nov 2, 2014 at 9:00

Halve means "divide by two" or "divide into two parts". Bisect also means "divide into two parts"; it is mostly used for geometry problems.

Trisect means "divide into three parts".

Quarter means "divide by four" or "divide into four parts".

Tithe means "a tenth of" or "give a tenth of the net increase of". Decimate means "kill a tenth of".

• Trisect is a word used almost exclusively in a mathematical or geometric sense, more so even than bisect. It would be unusual to see it in common language. "Divided/cut/separated into thirds" or "divided/cut/separated into three {nouns}" would be far more likely to be used. Nov 3, 2014 at 4:35

It's not a common usage, but OED has this definition...

third (verb, transitive, defn 1)
To divide (anything) into three equal parts; to reduce to one third of the number or bulk.

In fact, it's so uncommon it's probably a bad idea to use it at all with that sense. All native speakers would understand the intended meaning in context, but most would probably think it "ungrammatical". To the extent that to third is used as a verb at all, it's usually in the sense of OED's next definition...

third (defn 2)
To speak in favour of (a motion, proposition, etc.) as third speaker; to support the seconder.

But it's worth noting that even the above is sufficiently rare that this speaker gets a laugh for using it, and in the written form, "third" is placed in scare quotes to acknowledge an unusual / non-standard usage.

There are two of us, so I'll halve the cake　　　　　　　　(we get [a] half each)
??There are three of us, so I'll third the cake 　　　　　　(we get a third each)
There are three of us, so I'll cut the cake into three　　 (far more likely)
There are four of us, so I'll quarter the cake 　　　　　　(we get a quarter each)

I don't think there are any valid verb forms beyond quarter = divide into four parts.

• Its interesting how "a quarter" sounds better than "a third", maybe because i didnt hear it much. Even tough the same "pattern" apply for: quarter or half, a third doesnt just sound that good. Nov 2, 2014 at 15:06
• @Alpha2k: I don't think any native speakers would see anything remotely unusual about the noun form a third. I assume the reason we think the verb form sounds odd is because we rarely hear it. Mainly because dividing things into three happens less often than dividing into two or four parts, but perhaps also because a long time ago people might have used the now-obsolete tripart where they needed a verb form. Nov 2, 2014 at 15:22