Why are things worn below the waist called a pair of something (pants, socks, underwear) but things worn on the upper body are called a single item (sweater, bra, shirt)?

It's called an 'idiom.'

Originally, pantaloons were made as two separate garments, one for each leg, as a means of keeping the legs warm beneath a gown without interfering with visits to the lavatory, or as additionally layers to go over the legs of trousers, similar to 'chaps,' to provide additional warmth of protection. Eventually they were stitched together, but retained the erroneous title of a "pair of pantaloons," which ultimately became a "pair of pants."

G'day ABnormally Happy,

Thank you for your question.

According to Straight Dope "First of all, let's note there is a class of objects that are thought to consist of two independent but connected parts, usually identical or at least similar to each other. In addition to pants and trousers, there are eyeglasses, scissors, tweezers, shears, pliers, and so on.

The terms for these objects are always plural in form, and they are usually referred to as "a pair of ...." This usage goes back to at least 1297 AD, when we have the expression "a peire of hosen."

The implication is that the two parts are separable in some sense, and in fact a pair of hose can often mean two separate pieces. (True, you can't separate tweezers, but I never claimed the English language was rational.)"

The usage is pair of panties and could be pair of brassiere but Brassiere comes from French with different usage.

I have attached the source.

Regards

Straight Dope
http://www.straightdope.com/classics/a2_249b.html

Possibly because you wear them on a "pair" of legs.

From: World Wide Words

"Before the days of modern tailoring, such garments, whether underwear or outerwear, were indeed made in two parts, one for each leg. The pieces were put on each leg separately and then wrapped and tied or belted at the waist (just like cowboys’ chaps). The plural usage persisted out of habit even after the garments had become physically one piece. However, a shirt was a single piece of cloth, so it was always singular."
http://www.worldwidewords.org/qa/qa-pai1.htm

So where did you say you wore your earmuffs? I almost asked about your gloves, then realized they are below the waist sometimes. Most folks wear two shoes and two socks so the pair is obvious. The reason for a "pair of" and the use the plural pants, slacks, trousers etc instead of just pant is most often found in English, most other languages now use a singular form. Occaisionaly you will hear someone, usually a fashion advisor or commentator, use a term like " a trouser of wool", mostly they are just trying to sound oh so smart.
The reason for the plural and the pair goes way back to when they actually were a pair instead of one garment. Similar to the leggings worn by some Amerindians. Each leg was seperate and put on individually. At the top they were fastened to a belt or some other garment by straps, strings, laces, buttons or some other arraingement. Then the gap at the top was covered with something like briefs or just a long shirt, tunic, or other garment. Especially around Elizabethan times there was a real complicated arraingement of garments all tied together and held together with laces woven all around and through holes and eyelets. It was so complicated that men carried around a special tool to help with all the inlacings and relacings, it was called a bodkin. That's why pants, trousers, breeches etc were classified as pairs. Later when somebody had the bright idea to connect the legs the use of pair just continued.
The use of pair for underwear probably comes from the similarity of having two legs or at least two leg holes or just from being in the same general area of the body. However, up into the early 20th century, when women still wore layers and layers of clothes, they usually wore underpants that were two pieced, or at least two seperate leg pieces suspended from some sort of support. The two pieces were wide enough to have a more or less secure (usually less) ovelap at the center. It was hard enough just to get all those clothes up and out of the way when visiting the neccessary, much less have to remove and replace the underpants. Later when the pants became more unified the gap remained. Which does not imply that Queen Victoria was a kinky lady even if she did wear open crotch knickers.
The bra is a more modern invention and never was two seperate pieces so women have never worn a pair of bras. Tho I have seen a few women who could use the extra support.

I don't know--i have always wondered that myself--annoys the hell out of me

Because there are two legs on the pants. I think that's right, but actually, it doesn't make sense to me either

why is a tooth brush not called a teeth brush?
who cares

Gloves? Spectacles? Brass Ears? sure there are more. I always look for my Brass Ears don't you?

Eyeglasses are a pair of glass lenses for the eyes. The odd thing is not that it's a plural word; the odd thing is that the Germans make a singular out of it (Brille). Ukrainians and some other Slavonics call them "oculars" and Russians call them "little eyes".

The first eyeglasses weren't plural. You would buy a single lense for one eye and it would be called a "monocular," shortened to "monocle" and quite naturally an "eye glass." In "Treasure Island," which is not that old a book, a telescope is called a "good glass".

If you go back far enough in the history of English you'll discover similar reasons for other words. We wear a pair of pants, which used to be "pantaloons". Go back farther and men didn't wear pants; they wore "breeches" and a pair of tied-on "leggings".

So many things occur in pairs (because of bilateral symmetry and other dualities in the world we live in) that some languages, such as classical Greek and Biblical Hebrew, have two plurals--one plural just for pairs of things, and another for more than two.

Because the pants feature a pair of leggings.

We say pant as in "one of my pant legs".....so I dunno...LOL! I guess because there is two legs? Hence, the pair? I dunno, just guessing here, haha.

Yes it should be

why is it a pair of panties/underwear

One part for each leg, then belted in the middle, somewhat like chaps on cowboys. Over time, they ended up as one piece of clothing, but the habit of referring to the old "pair" persisted.

Maybe because they have two legs in them for your pair of legs. I honestly have no idea though. I just call them pants.

I think because there are two legs. Look at the new PANTS I bought.

has it got something 2 do with the fact that there is 2 legs which makes it a pair...(of legs) thats why its a pair of pants because a pair of legs just sounds daft!

I can maybe understand a pair of pants since there are two legs... but a pair of underwear? It seems a bit weird.

and what do you call a pair of bikini?

because it caters for two butt cheeks i guess!

I think it stems from the fact that a pair of pants is technically a pair of pants legs. A pair of underwear is kind of like pants without actual legs. But it didn't used to always be that way.

And for another matter, why do they call underpants underwear, but not a bra--which is, techically, underwear. The very decription of underwear is clothing you wear UNDER your clothing.

Because really, they are one thing, but there's 2 parts. Pants=2 legs, shoes=2 shoes, glasses=2 lenses, underwear= 2 holes for your pair of legs. Most of the time "pair" is a term used for "twos."

i'll have to put on my pair of spectacles and study that question