Although most modern audiences have heard the taxi horns using the notes A, B, C, and D, it had been Gershwin's intention to use the notes A4, B4, D5, and A3.
ɔːlðˈoʊ mˈoʊst mˈɑːdɚn ˈɔːdiənsᵻz hæv hˈɜːd ðə tˈæksi hˈɔːɹnz jˈuːzɪŋ ðə nˈoʊts ˈeɪ, bˈiː, sˈiː, ænd dˈiː, ɪt hɐdbɪn dʒˈɜːʃwɪnz ɪntˈɛnʃən tə jˈuːz ðə nˈoʊts ˌeɪ fˈoːɹ, bˈiː fˈoːɹ, dˈiː fˈaɪv, ænd ˌeɪ θɹˈiː.