ðɪs mˈiːnz ðˈæt, ɪf ɐn ˈɛdʒ ˈiː ɪn wˈʌn dᵻɹˈɛkʃən hɐz lˈeɪbəl dʒˈiː ɐ ɡɹˈuːp ˈɛlɪmənt, ðˈɛn ɪnðɪ ˈʌðɚ dᵻɹˈɛkʃən ɪt hɐz lˈeɪbəl dʒˈiː mˈaɪnəs wˈʌn.
This means that, if an edge e in one direction has label g (a group element), then in the other direction it has label g −1.