It is historically considered the first "sixth grade" in six-tier scale of alpinistic difficulties proposed by Willo Welzenbach (corresponding to 5.9).
ɪɾ ɪz hɪstˈɔːɹɪkli kənsˈɪdɚd ðə fˈɜːst "sˈɪksθ ɡɹˈeɪd" ɪn sˈɪkstˈɪɹ skˈeɪl ʌv ˌælpɪnˈɪstɪk dˈɪfɪkˌʌltiz pɹəpˈoʊzd baɪ wˈɪloʊ wˈɛlzənbˌɑːx kˌɔːɹɪspˈɑːndɪŋ tə fˈaɪv.nˈaɪn.