Chomsky's advisor, Zellig Harris, took transformations to be relations between sentences such as "I finally met this talkshow host you always detested" and simpler (kernel) sentences "I finally met this talkshow host" and "You always detested this talkshow host." A transformational-generative (or simply transformational) grammar thus involved two types of productive rules: phrase structure rules, such as "S → NP VP" (a sentence may consist of a noun phrase followed by a verb phrase) etc., which could be used to generate grammatical sentences with associated parse trees (phrase markers, or P markers); and transformational rules, such as rules for converting statements to questions or active to passive voice, which acted on the phrase markers to produce other grammatically correct sentences. Hjelmslev had called word-order conversion rules "permutations". (For more details see the Transformations section below.)