When two words are frequently used in speech and writing, they will be used closer and closer together over time. Usually, this happens in speech and then translates to writing, where compounds become recognized as words within their right.
An excellent example is the closed compound word ‘notebook.’ Before ‘notebook’ was a word, we would have used something like “a book to write notes in.”
Likely, this would have been shortened to an open compound, ‘notebook.’ Words like this would then become hyphenated compound words like note-book until finally, we arrive at the word ‘notebook,’ which we use in the English language today.
We can even see how the English language evolves by comparing compound words that should be similar.
The words ‘backyard’ and ‘backseat’ are closed compound words, but the words ‘front yard’ and ‘front seat’ are open compound words. Yet they both outline either a specific portion of the yard or the seat position in a vehicle.
However, sometimes compounds are formed because of a gap in our language. For example, the word ‘football’ would have been created out of necessity as the game itself would not have had a name to begin with. This compound is thought to be very literal, as it’s a compound of ‘foot’ and ‘ball’ from kicking the ball with the foot.