information:classification, structural:scholar_edition
Vocal category

Callas' voice has been difficult to place in the modern vocal classification or Fach system, especially since in her prime, her repertoire contained the heaviest dramatic soprano roles as well as roles usually undertaken by the highest, lightest and most agile coloratura sopranos. Regarding this versatility, Serafin said, "This woman can sing anything written for the female voice". Michael Scott argues that Callas' voice was a natural high soprano, and going by evidence of Callas' early recordings, Rosa Ponselle likewise felt that "At that stage of its development, her voice was a pure but sizable dramatic coloratura —that is to say, a sizable coloratura voice with dramatic capabilities, not the other way around." On the other hand, music critic John Ardoin has argued that Callas was the reincarnation of the 19th-century soprano sfogato or "unlimited soprano", a throwback to Maria Malibran and Giuditta Pasta, for whom many of the famous bel canto operas were written. He avers that like Pasta and Malibran, Callas was a natural mezzo-soprano whose range was extended through training and willpower, resulting in a voice which "lacked the homogeneous color and evenness of scale once so prized in singing. There were unruly sections of their voices never fully under control. Many who heard Pasta, for example, remarked that her uppermost notes seemed produced by ventriloquism, a charge which would later be made against Callas". Ardoin points to the writings of Henry Chorley about Pasta which bear an uncanny resemblance to descriptions of Callas:

There was a portion of the scale which differed from the rest in quality and remained to the last 'under a veil.'...out of these uncouth materials she had to compose her instrument and then to give it flexibility. Her studies to acquire execution must have been tremendous; but the volubility and brilliancy, when acquired, gained a character of their own... There were a breadth, an expressiveness in her roulades, an evenness and solidity in her shake, which imparted to every passage a significance totally beyond the reach of lighter and more spontaneous singers... The best of her audience were held in thrall, without being able to analyze what made up the spell, what produced the effect—as soon as she opened her lips.

Callas appears to have been in agreement not only with Ardoin's assertions that she started as a natural mezzo-soprano, but also saw the similarities between herself and Pasta and Malibran. In 1957, she described her early voice as: "The timbre was dark, almost black—when I think of it, I think of thick molasses", and in 1968 she added, "They say I was not a true soprano, I was rather toward a mezzo". Regarding her ability to sing the heaviest as well as the lightest roles, she told James Fleetwood,

It's study; it's Nature. I'm doing nothing special, you know. Even Lucia, Anna Bolena, Puritani, all these operas were created for one type of soprano, the type that sang Norma, Fidelio, which was Malibran of course. And a funny coincidence last year, I was singing Anna Bolena and Sonnambula, same months and the same distance of time as Giuditta Pasta had sung in the nineteenth century... So I'm really not doing anything extraordinary. You wouldn't ask a pianist not to be able to play everything; he has to. This is Nature and also because I had a wonderful teacher, the old kind of teaching methods...I was a very heavy voice, that is my nature, a dark voice shall we call it, and I was always kept on the light side. She always trained me to keep my voice limber.