The words legato and sostenuto mean, respectively, to connect and to sustain. Whether a song is fast or slow, sad or gay, simple or difficult, it is necessary for its successful rendition to go from one tone to the next smoothly and with accurate intonation. To do this the voice must be freely produced and the breath ample to sustain the tone and carry the phrase to the end.

Closely associated with legato style is portamento, or carrying the voice from one pitch to another with a delicate glide. It is distinguished from legato by the fact that pitches between the limiting tones are lightly sounded. The portamento must be done lightly, smoothly, and exactly, with no suggestion of force. It should be employed only at appropriate places in the composition and should never even faintly suggest what is commonly known as scooping often heard in untrained and inartistic singing. If the composer wishes this effect, he usually indicates it by using a slur, or curved line, or the phrase "portando la voce" (carrying the voice).
The opposite of legato and sostenuto singing is staccato’ or detached style of vocalization, where each tone is attacked, then released quickly so that definite intervals of silence occur between them.

Smooth;’ flowing delivery of voice, or legato singing, was the foundation of the Italian bel canto art of the seventeenth century. It is as important in the equipment of the singer today, for upon it depend all vocal graces.