In English, certain terms can be confusing because they sound like they could be either one word or two. Take, for instance, the common expression “made for each other.” Despite frequent misspellings, there is only one way to write the reciprocal pronoun “each other” — with a space between the words. This makes it an “open” compound word. In contrast, compound words written without spaces are known as “closed” compounds. Compound words function in all parts of speech: nouns, verbs, adjectives, adverbs, and even prepositions. There aren’t any universal grammar rules for compound words, so their usage often comes down to meaning, context, and convention.
Speaking of “made for each other” relationships, while “boyfriend” and “girlfriend” are written as one word, “best friend” is two words. Often, when a compound noun contains an adjective (like “best”), it’s written as two words, whereas compound nouns featuring two nouns are more commonly written as one word (“boyfriend”). While there are a few loose guidelines, hard rules are rare. For example, “blackboard,” “smartphone,” and “greenroom” are one-word adjective-noun compounds. Similarly, “workday” and “weekday,” which can function as adjectives or nouns, are written as single words.
School-related compound words are just as unpredictable. “High school” is an open compound, but “schoolteacher,” “schoolwork,” and “schoolroom” are closed. To remain inconsistent, the terms “school year” and “school day” are two words. Compounds that function as pronouns form another murky category. While “each other” and “no one” remain two words, “someone,” “anyone,” and “everybody” are closed. In these situations, the categories and functions of the terms have little impact on whether or not they are written as one or two words.
As you can see, we don’t have much help in the form of a handy rule that will tell you when a compound is one word or two. The safest bet is to consult a dictionary when unsure. In the end, compound words often say “never mind” (yes — two words) to typical grammar expectations, leaving writers to memorize their favorite words and look up the rest.
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