The literal meaning of the Italian word tessitura is "texture," or "weave," as in the weave of a fabric. In music for voice, the term refers to where a piece "lies" for the voice how high or low the music is, not at its highest and lowest points, but in general. The tessitura of a piece determines what voice type should sing it. For example, a song or operatic role may contain notes that are singable by either a soprano or a mezzo-soprano: the lower-voiced mezzo can sing even the highest notes the role requires, and the soprano can reach the lowest. But does the role have a high tessitura, better suited to a soprano, or a low tessitura, suited to a mezzo? For whom would the role be a strain and for whom would it lie for the most part in a vocal comfort zone? Who would sound better singing it? It's not a matter of who can reach the notes, or of an occasional stretch in one direction or another, but rather of making sure that the person singing will be using the strongest and most beautiful part of her voice most of the time.
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