Grammar Girl here.
Today's topic is units of measure—those words and symbols that come after numbers.
This first point might seem self-evident: you put a space between the number and the unit of measure. Even though it seems obvious, I'm telling you because I've seen people do it wrong, and there are also exceptions to the rule.
For example, if you want to say you threw a ball 100 feet, there is a space between the number 100 and the word feet. Obvious, right? But what if you abbreviate feet to ft*? It's the same rule: there's a space between 100 and ft, and if you just think of the abbreviation as the word, it should be easy to remember, but I've seen them squished together without a space many times.
Sometimes I think people get confused because there are at least two exceptions to that rule—percent and degrees—but it helps to remember that these exceptions occur when you are using symbols instead of abbreviations. For example, if you are writing out the words 100 degrees, there is a space between the number 100 and the word degrees, but if you are using the degree symbol, that little superscript circle, you butt it right up against the number without any spaces in between (100°—no space). The same holds true if you want to use the percent sign. Even though you use a space when you are writing out the word percent, if you use the percent sign, it goes right after the number without any spaces (100%—no space).
For the next point, let's go back to talking about feet. The singular is obviously foot, and the plural is usually feet, except when you are using it as a compound modifier. You would say that Squiggly climbed a tree that was 10 feet tall†; feet is plural there. But you don't say Squiggly ran up a 10-feet tree—you say Squiggly ran up a 10-foot tree. Foot is plural when it's part of a compound modifier. That's also true for other units of measure like inches and pounds. Squiggly was running from a 100-pound monster, and he nearly lost his 10-inch tentacles.
Similarly, the abbreviations for units of measure are the same whether the units are singular or plural. the abbreviation is ft. whether it is foot or feet. Occasionally you will see an s after the abbreviation for pounds, lbs., but it isn't necessary.
Style guides are divided about whether you need to put a period after the abbreviations for English units of measure like feet, inches, and pounds‡. So it's up to you to pick a style and use it consistently. In general, it's more common to use periods in the U.S. than in Britain (1). With the metric system, or more formally the International System of Units, you never use a period after the abbreviations (2).
And here's a bonus: Have you ever wondered why pounds is abbreviated as lb instead of pd or something that relates to how it is actually spelled? As is often the case, it has to do with Latin. The abbreviation lb stands for the Latin phrase libre pondo, which means "pound of weight." There are some really extensive explanations on the web, so I put a few links in the blog.
Math Forum Explanation
World Wide Words Explanation
* Most style guides recommend writing out units of measure (e.g., feet instead of ft.) unless you are writing a technical or scientific document. Abbreviations are acceptable in tables.
† The Oxford Dictionary of English Usage states that “foot” can also be used in this sentence, as in “Squiggly climbed a tree that was 10 foot tall.” It notes that this use is correct, but less common.
‡ For example, The Chicago Manual of Style (14th edition) states that abbreviations are usually written without periods (p.477), but goes on later to give examples with periods. The Modern Language Association Handbook gives examples with periods, but notes that the trend in abbreviations is away from using periods.
That's all.
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