A singer will choose a repertoire that suits their instrument. Some singers such as Enrico Caruso, Rosa Ponselle, Joan Sutherland, Maria Callas, Jessye Norman, Ewa Podleś, or Plácido Domingo have voices that allow them to sing roles from a wide variety of types; some singers such as Shirley Verrett or Grace Bumbry change type and even voice part over their careers; and some singers such as Leonie Rysanek have voices that lower with age, causing them to cycle through types over their careers. Some roles as well are hard to classify, having very unusual vocal requirements; Mozart wrote many of his roles for specific singers who often had remarkable voices, and some of Verdi's early works make extreme demands on his singers.