Thus the most frequent word will occur about twice as often as the second most frequent word, three times as often as the third most frequent word, etc. For example, in one sample of words, the most frequently occurring word accounts for nearly 7% of all the words (69,971 out of slightly over 1 million). True to Zipf's Law, the second-place word "of" accounts for slightly over 3.5% of words (36,411 occurrences), followed by "and" (28,852). Only 135 vocabulary items are needed to account for half the sample of words.
The same relationship occurs in many other rankings, unrelated to language, such as the population ranks of cities in various countries, corporation sizes, income rankings, etc. The appearance of the distribution in rankings of cities by population was first noticed by Felix Auerbach in 1913.
It is not known why Zipf's law holds for most languages.
- Zipf George K. 1935. The psychology of language. Houghton-Mifflin.
- Zipf George K. 1949. Human behavior and the principle of least effort. Addison-Wesley.
- Auerbach F. 1913. Das Gesetz der Bevölkerungskonzentration. Petermann’s Geographische Mitteilungen 59, 74–76
- Brillouin, Léon  2004. La science et la théorie de l'information.