What is the primary characteristic of compound words?

The primary characteristic of compound words is that they consist of two words combined. Compound words are formed when two independent words are joined together to create a new word with a distinct meaning. For instance, "tooth" and "brush" come together to form "toothbrush." This blending of meanings from the individual words is what distinguishes compound words from other types of word formations.

In contrast to the other choices, the focus on phonetic sounds (the first option) does not specifically address the nature of compound words themselves but rather the sounds within words. The requirement for short vowel sounds (the third option) is inaccurate, as compound words can contain both short and long vowel sounds depending on the individual words that are combined. Lastly, the inclusion of prefixes and suffixes (the fourth option) pertains to derivational morphology rather than the core concept of compound words, which strictly involves the combination of two complete words. Therefore, the defining feature here is the combination of two words to create a new word.