As you can probably see from our title, in most cases mother-in-law is indeed hyphenated. It is a hyphenated compound noun (i.e. two nouns together for the one entity). This means all compound nouns are hyphenated, right? If only it were so simple. The rules for compound nouns and hyphens use an erratic logic which has little consistency. Unfortunately, this is something English loves to do with its grammar rules. While much of English grammar has a strict logic, there is a sizable amount which doesn't. This is why oneHOWTO not only answers 'is mother-in-law hyphenated?', but we find out more about how to use these often capricious little punctuation marks.
Which words are hypehnated?
Like all the in-laws, mother-in-law is always hyphenated, same as father-in-law or sister-in-law. Perhaps a better question is why is mother-in-law hyphenated? Unfortunately, there is no concrete answer to this question. To get some idea we need to look at hyphen use in general.
Compound nouns are not the only words for which hyphens are used. They are also common in compound adjectives. An example of this might be three-day weekend. The hyphen here brings the two words together,doing so because it reduces confusion.
There are two main types of adjective, attributive and predicative adjectives. This is a fancy way of saying the placement of the adjective in reference to the noun.
Attributive adjectives
With attributive adjectives, the adjective comes before the noun. If using a two word compound attributive adjective, we hyphenate the words.
- 'She was in a highly-toxic relationship'
Predicative adjectives
With predicative adjectives, the adjective comes after the noun and there is no need to hyphenate.
- 'Her relationship was highly toxic'
There are times when a hyphen is used in a compound adjective whether or not it comes before the noun. This is for adjectives which come hyphenated as standard. A good example is English-speaking.
- 'The English-speaking students were the worst in class'
- 'In class, worst students were English-speaking'
Knowing which compound adjectives are hyphenated as standard is something which comes with practice. Many native English speakers will know which are in common usage simply because they have grown up with them. However, many don't and hyphens often get added or subtracted by even the most articulate of people.
We often use verbs as adjectives, especially superlatives and comparatives. These are the words which we use for the best version (superlative) and two compare two nouns (comparative). When these words are used with a participle of a verb and this participle is being used as an adjective, we do not use a hyphen:
- 'Adele had the fastest selling album last year'
In this sentence selling is a verb which is being used as an adjective. When a verb in the comparative or superlative is not being used as an adjective, you do hyphenated:
- 'The worst-case scenario is we have to drive home'
Here case is a noun being used as an adjective and therefore we do hyphenate.
With compound nouns, we often use hyphens to change the meaning of a sentence. For example:
- 'He was a great uncle for the first time'
This sentence means that for the first time, he achieved greatness.
- 'He was a great-uncle for the first time'
This sentence means the birth of a new child made them a great-uncle for the first time.
Also, remember that hyphens are not used for modal verbs. E.g. build up, cast away. If you use a hyphen then you turn them into compound nouns:
- 'A build up of fluid'
- 'He was a cast-away after the boat sank'
Why is mother-in-law hyphenated?
The main reason for hyphenating family names is to keep things simple. With compound nouns, if you have more than two words it can get confusing without the use of hyphens. For example, if we didn't use hyphens for mother-in-law we can get confused looking sentences like this:
- 'My father in law was in deep trouble when he saw my mother in law in the house'
Using hyphens can tidy up sentences like these:
- 'My father-in-law was in deep trouble when he saw my mother-in-law in the house'
We use hyphens for all in-law nouns:
- in-laws
- mother-in-law
- father-in-law
- sister-in-law
- brother-in-law
- cousin-in-law
- aunt-in-law
- uncle-in-law
Note that the in-law term is technically used for cousins, aunts and uncles, but very rarely.
We wish it could be simple enough to say that we use hyphens when speaking about family and extended family. It is the case for some of them. Examples include when we talk about great-grandparents:
- great-grandfather
- great-great-grandmother
However, if you have a sibling through marriage, but not through blood, then hyphens do not get used. This is the case when your parent marries someone else who already has children. These are known as stepsiblings from stepparents in your stepfamily. For some reason, these compound nouns are put together with no space or hyphen:
- stepfather
- stepmother
- stepbrother
- stepsister
However, some stepfamilies become so close, they also want to incorporate the brothers and sisters of stepparents. If this happens, they might want to call then a step-uncle or step-aunt, so we do use hyphens.
When using family compound nouns, sometimes you neither use a hyphen nor do you use a conjunction (joining the words together). These are the cases when you get into cousins:
- first cousin
- second cousin
- third cousin
Our final family compound noun examples are used for when you have siblings who are born to one biological parent, but not the other, e.g. your mother and your stepfather. When this happens you have a half-sister or half-brother. Do you hyphenate these words? The answer depends on where you are from.
In British English, the words for half-siblings are always hyphenated. However, in US English, the term half sibling without a hyphen is used interchangeably. In fact, it appears to be more common to not use a hyphen in US English.
Hyphen or en dash?
While you might think that all the horizontal lines in typography are hyphens, this is not so. However, when using compound nouns and adjectives you will always use the hyphen. This is the shortest line on your keyboard and it has two types:
Suspended hyphen
This is the hyphen between two words and it is 'suspended' because it is held up by these words.
- 'The anti-fascists lead a peaceful demonstration'
Hanging hyphen
A hanging hyphen has a word before it, but not after. It is used most often when writing two prefixes for the same word:
- 'The anti- and pro-fascists did not engage during the meeting'
En dashes (–) are not a hyphen. There are many different dashes which can be explained at another time, but the en dash is important when using compounds. They are longer than a hyphen and get their name from being the width of a typesetter's letter 'N'.
En dashes are used when the sentence has a compound noun in the adjective for a word:
- 'The mother-in-law–table decorations were the nicest'
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