If you are reading a book, you might find it odd how hyphens are used. In one case, the author describes a character has having green eyes, but in another, says that the character is green-eyed.
While these mean the same thing and have the same words in them, why does one have a hyphen and the other does not? How are the two phrases different, and how can you apply that to your own writing?
A hyphen is a punctuation mark (-) that is used to connect words. This is useful to clearly show which words are related in a sentence, and which words are not. They are often used with numbers and in compound adjectives, also known as adjective phrases.
There is no space between the symbol and the letters on either side of it, so it is shorter than the M dash, which is used in a similar way that a comma is to separate thoughts in a sentence.
Hyphens in Compound Adjectives
The flexibility of English allows for word combinations that can describe things that we do not have a single specific word for.
When you are describing a girl who is 7 years old, for example, you would have to put those words all together. In that case, you need to use “7 years old” as an adjective describing her.
To make sure that you can tell the difference between the adjective and the noun that it describes, the hyphen comes into play. For this case it would be like this: 7-year-old girl. The hyphens go between the words that are meant to be the adjective.
You can, therefore, string together a substantial list of words to make an adjective. Most of the time the phrases are limited to 2 or 3 words, though.
Some examples of compound adjectives include:
For each of these examples above, you can think about the nouns that each adjective phrase would describe. For example, the 24-hour could mean a day, a schedule, a routine, a hotline, or something else. You often hear about companies that are forward-thinking, or innovation-driven.
They are often used with numbers and in compound adjectives, also known as adjective phrases. There is no space between the symbol and the letters on either side of it
In some cases, the addition of a hyphen is not necessary. If the reader can understand your meaning without the hyphen, it is not a problem. In some cases, though, the hyphen can completely change the meaning of a phrase. Take the following two examples:
- Heavy metal detector
- Heavy-metal detector
In the first example without the hyphen, the phrase can be broken down after the first word. This is a metal detector that is heavy. However, this is not the same meaning as the second example.
Because there is a hyphen between the first two words in this example, you know that heavy and metal go together. For this, you are looking at a detector that looks for heavy metals. This is quite a difference.
Usage of Compound Adjectives
When you have a compound adjective, you almost always use it at the beginning of a noun. At the end of the noun, the phrase would more likely be connected in a different way (with a linking verb, for example) that would make the relationship between the words in the compound adjective and the words in the noun clearly separate.
The hyphens go between the words that are meant to be the adjective. You can, therefore, string together a substantial list of words to make an adjective.
Take this example:
- The use of evidence-based medicine should have been implemented years ago.
In this sentence, the compound adjective comes before the noun, and to cut down on possible confusion around it, there is a hyphen to join the words that make up the adjective (evidence and based).
- The use of medicine that is evidence-based should have been implemented years ago.
Notice that, while this sentence is also grammatically correct, it feels more drawn out. Rather than describe the medicine directly, it requires a few extra words to talk about it. The use of the compound adjective works much better. The alternative to this is a simpler sentence:
- The medicine is evidence-based.
- The medicine is based on evidence.
Both of these options are clear and correct.