United States Supreme Court decisions in Baker v. Carr (1964) required that both houses have districts established on the basis of population, and redistricted after each census, to implement the principle of "one man, one vote".
juːnˈaɪɾᵻd stˈeɪts suːpɹˈiːm kˈoːɹt dᵻsˈɪʒənz ɪn bˈeɪkɚ vˈiː. kˈɑːɹ nˈaɪntiːnhˈʌndɹɪd sˈɪksti fˈoːɹ ɹᵻkwˈaɪɚd ðæt bˈoʊθ hˈaʊzᵻz hæv dˈɪstɹɪkts ɪstˈæblɪʃt ɔnðə bˈeɪsɪs ʌv pˌɑːpjʊlˈeɪʃən, ænd ɹˌiːdˈɪstɹɪktᵻd ˈæftɚɹ ˈiːtʃ sˈɛnsəs, tʊ ˈɪmplɪmənt ðə pɹˈɪnsɪpəl ʌv "wˈʌn mˈæn, wˈʌn vˈoʊt".