In most cases these developed as true extensions of ASCII, leaving the original character-mapping intact, but adding additional character definitions after the first 128 (i.e., 7-bit) characters.
ɪn mˈoʊst kˈeɪsᵻz ðiːz dɪvˈɛləpt æz tɹˈuː ɛkstˈɛnʃənz ʌv ˈæski, lˈiːvɪŋ ðɪ ɚɹˈɪdʒɪnəl kˈæɹɪktɚmˈæpɪŋ ɪntˈækt, bˌʌt ˈædɪŋ ɐdˈɪʃənəl kˈæɹɪktɚ dˌɛfɪnˈɪʃənz ˈæftɚ ðə fˈɜːst wˈʌnhˈʌndɹɪd twˈɛnti ˈeɪt ˈaɪ.ˈiː., sˈɛvənbˈɪt kˈæɹɪktɚz.