Catchiness is how easy it is for one to remember a song, tune or phrase. It is often taken into account when writing songs, catchphrases, advertising slogans, jingles etc. Alternatively, it can be defined as how difficult it is for one to forget it. Songs that embody high levels of remembrance or catchiness are known as "catchy songs" or "earworms". While it is hard to scientifically explain what makes a song catchy, there are many documented techniques that recur throughout catchy music, such as repetition, hooks and alliteration. Selling Sounds: The Commercial Revolution in American Music says that "although there was no definition for what made a song catchy, all the songwriting guides agreed that simplicity and familiarity were vital".