In a perfectly phonemic orthography, there would be a consistent one-to-one correspondence between the letters and the phonemes so that a writer could predict the spelling of a word given its pronunciation, and a speaker would always know the pronunciation of a word given its spelling, and vice versa.
ɪn ɐ pˈɜːfɛktli fɑːnˈɛmɪk ɔːɹθˈɑːɡɹəfi, ðɛɹ wʊd biː ɐ kənsˈɪstənt wˈʌntəwˈʌn kˌɔːɹɪspˈɑːndəns bᵻtwˌiːn ðə lˈɛɾɚz ænd ðə fˈoʊniːmz sˌoʊ ðˌæɾɚ ɹˈaɪɾɚ kʊd pɹɪdˈɪkt ðə spˈɛlɪŋ əvə wˈɜːd ɡˈɪvən ɪts pɹənˌʌnsɪˈeɪʃən, ænd ɐ spˈiːkɚ wʊd ˈɔːlweɪz nˈoʊ ðə pɹənˌʌnsɪˈeɪʃən əvə wˈɜːd ɡˈɪvən ɪts spˈɛlɪŋ, ænd vˈaɪs vˈɜːsə.