Her voice is an alto; she can cover all the octaves between an alto and a soprano. According to the BBC, "Such a range is so unusual because the human voice is very limited and we use only a small part of it. This makes it very difficult to expand the vocal range and articulate such high notes. Exploiting the range we do have comes down to physical aspects, like body structure and the length of vocal cords, technique and psychological attitude." Gillyanne Kayes, a vocal coach/researcher states, "The young Mariah Carey used to sing in a lowish voice and then these stratospheric notes would sound more and more like whistling. It takes a high degree of skill to sing across a range. The colour, or timbre, of a voice is not the same all the way through."