In Australian English, we'd refer to using feet, inches, pounds, ounces and the like as "imperial units". In everyday conversation, do Americans refer to their non-metric units as "imperial", even though it's slightly different to the non-metric units Australians use? If not, what do they use?
Learn English – How do Americans refer to their non-metric system in everyday circumstances
american-english
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Anyway, when talking to British people do I really have to expect that they don't know what I'm talking about when using the word rubber instead of condom?
Yes. That definition of rubber is not really used in the UK. Some people would know it from personal experiences of Americans. Like me. I know it because I have met some Americans in the UK and this word came up in the context of meeting one of them: in primary school, there was an American pupil in the last year. This definition would not generally, be reliable in the UK.
And when talking to American people, should I avoid rubber and use eraser instead whenever I expect it might sound ambiguous? To be more concrete: the OALD designates rubber (as eraser) as British English, thus do I have to suspect that an American may not be aware of that meaning?
That would be the most reliable approach. As you can see from this discussion, there are some Americans who are aware of the British meaning of rubber but it is not used in the USA. Because of this, it is unreliable in the USA and many Americans might not know this meaning. When talking to Americans, it would be best to use the American word, eraser.
Finally, when talking to both an American and a British at the same time, which words do I use for condom and eraser so that both will understand me correctly?
For a condom, just use the word condom. This is the proper word used in the UK and the USA. As the following links show, using the word rubber to mean a condom, is American slang:
US slang for a condom (Cambridge Dictionaries Online)
[countable] American English informal a condom (Longman Dictionary)
And the USA has this slang for the word, as well as the word itself.
For the word eraser, there will have to be a different kind of answer. This is an example of, one of the many differences in American English. Because the word eraser is not generally, reliable in the UK, it is not certain that any, particular British person, will know its American meaning. Whether or not it will be understood, will depend on which, particular British people you talk to.
If you talk to an American person and a British person at the same time, just use the word condom. It is understood in both countries.
Regarding eraser, you have a few possibilities. If you find out first, that one of the two knows the word used in the other's country; you can use a sentence that has just that word. For example, if you knew that the American person knows the British meaning of rubber, you can use just that. It could be that you are writing with a pencil and want to change some of the writing. You could ask, "have you got a rubber?". If you don't know in advance whether one of the two knows the word used in the other's country; you would need to use a sentence that covers both words. For example, you could ask "have you got a rubber/eraser?". You could also find out by asking, when you talk to them.
It has that meaning in the US, although it's a bit jargon-y. Most people will say "plates", but most will understand "tags" at least in context. When you buy a car at the dealer you pay a "tag and title" fee through the dealer to have the car registered and receive a license plate, and the shops that exist in some states to handle registration paperwork outside of the DMV offices are usually called "tag and title service". You might also hear a policeman report something about "a white Chevy with Michigan tags."
Despite how it may seem, "tags" does refer to the license plate, not specifically to the sticker or decal that's attached to the plate to show that the registration is current. Wiktionary is in agreement with me on this one (ety 1, n. 8.)
Best Answer
In day to day conversations our system of units and measures are not called any thing. We don't have different conversions happening between systems, in most cases, and for us it just makes sense.
For a simple example there are 3 feet in a yard. But we never really do that conversion. It would be far more likely for me to ask for 99 feet of foo, than to try to ask for either 99 feet or 33 yards.
Same for cooking measures. We can convert cups to quarts to gallons and all, but usually we never have to. (and I think this is one of the more common conversions)
The only really common conversion that we do daily, is feet to inches. Even that is not really true, for a lot of people. If you try to buy window blinds for example, you measure with a measuring tape and record the inches, then in the store you shop by inches. Even if its 72 inches, that's still fine.
Having to convert from say feet to meters is extremely rare (on a large scale). If there are professions or common needs that require a lot of conversion there are charts, slide rules, and other aids. But even in those situations (most of the time) the metric units are the foreign ones.
Because we don't go around converting units very much, we really don't have a name for our set of units. There are some names that are in use when comparing the two systems of measures. (Please note these are generalizations)
Probably the most common is when working on cars and other such. Bolts and sockets, wrenches are either SAE (Society of Automotive Engineers) for US built cars, or Metric everywhere else.
Second to that is probably SI. For scientific things, including medicine, chemistry, experiments, etc. SI is used. But almost exclusively. So there are no "Customary Units" in use, most of the time.
When going over historic documents Imperial is some times used. It is also used incorrectly to contrast metric v.s. Imperial. But it's very important to note that US units of measure are not the same as Imperial units, and in some cases (most modern ones) Imperial is used incorrectly.
Customary, Traditional, and Standard terms are generally used in conversations that compare the two systems of measurements. But it can really depend on what circles you run in, which term you would use.
And finally, American is used frequently when asking someone to convert SI units to something more locally useful. "He was about 3 meters tall." "What's that in American?" "Almost 10 feet!"
As an aside, "Freedom Units" are a joke on "Freedom Fries" and are usually mentioned with scorn and laughter. I can't think of anyone that actually uses that term seriously.
So in day to day use I would say we just call them weights and measures. If we are comparing them to SI units then we may use a name, but that name is, in itself, not really standardized. The official standard name is United States customary system or United States customary units but that's almost as rarely used as "Freedom Units" (though "customary units" is a bit more common).