When you say pronounce some terms, they sound as if they are one word, and some are but others are not. Some are two words and some are two words with a hyphen, Why the variation? It is how words naturally evolve.

The English language allows for what is called free compounding, that is, putting two words together, particularly two nouns, to form a descriptive term. So let’s say I have a table where I keep my computer, and I refer to it as my computer table. You would understand what I mean, even though the term would not come to mind as quickly as, say, toolbox. You are not as accustomed to hearing computer table.

If manufacturers began making such tables, the term probably would become well known and soon you would retrieve it as a whole from your mental lexicon. According to Dana McDaniel, a linguist, dictionaries would probably write it as two words initially, then as a hyphenated term, and perhaps eventually as one word. That’s why teen age became teen-age and, as of a few years ago, teenage. Similarly, threshold was once thresh hold, although was all though, and until was on till before the 1300s, according to Geoffrey Pullum, a professor of linguistics at the University of Edinburgh and the author of The Cambridge Grammar of English.

The best suggestion is to keep a good stylebook and dictionary on hand. The best stylebooks are the Associated Press Stylebook, the Chicago Manual of Style and Usage, and the Gregg Reference Manual. Websters College Dictionary, fourth edition, or the American Heritage Dictionary are the most widely used. All of these resource books are updated periodically, particularly usage manuals, to keep pace with language changes.