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The Fog index bases its score on sentence length and the number of long words in a passage- more particularly, the percent of words of three or more syllables. The hills in Visual Fog are created by turning long words into their short-word equivalents.
These three sentences, for example, have the same Fog score, equivalent to that of a twenty-five word sentence without any long words:
His manner was not effusive.[5 words/1 word with more than two syllables]
He was pacing the room swiftly, eagerly, with his head sunk upon his chest and his hands clasped behind him. [20/1]
"How do I know that you have been getting yourself very wet lately, and that you have a most clumsy and careless servant girl?” [25/0]
Since these three sentences are Fog-equivalent*, the 3+ syllable word is equivalent to an additional twenty words for the five-word sentence, and to an additional five words for the twenty-word sentence.
*Fog equivalence would actually require each of these sentences to be the average, in terms of length and long words, for a larger passage.