Legato and portamento are the essential elements in singing that are similar but have different meanings and sounding results. There are many aspects that distinguish legato and portamento, but there is some debate in the literature about the use and definition of these terms. Vocal pedagogue Manuel Garcia II (1805-1906) indicates legato as slurred sounds that the air must be in continuous pressure to connect all the notes. He explains: 

To sing legato means to pass from one sound to another in a neat, sudden, and smooth manner, without interrupting the flow of the voice; yet not allowing it to drag or slur over any intermediate sound. In this case as with slurred sounds, the air must be subjected to a regular and continuous pressure, so as intimately to unite all the notes with each other.1

This definition of legato by Garcia is similar to pedagogue Francesco Lamperti’s (1811-1892) definition of portamento. Lamperti (1890) explained portamento as ‘passing from one note to the other by slurring the voice but in such a manner that the intervening notes are heard as little as possible’.2 Garcia explained con portamento ‘to slur is to conduct the voice from one note to another through all intermediate sounds.’3 Portamento is most similar to legato in the manner in which it allows for continuity of tone and rhythmic flexibility, portamento occurs between any two notes either ascending or descending, or portamento can appear as a scoop such as with slides over a repeated pitch or slides into the beginning of a phrase from silence.  This continuity of tone can cause an early or late arrival on different melodic notes in relation to the accompaniment. As a consequence, portamento will blur the exact rhythmic and location of pitches.  

          Although Lamperti writes: ‘Legato means passing from one note to another quickly, so that the voice does not dwell upon the intervening notes just as if it were executed upon a piano or any other keyed instrument, his statement does not distinguish any concrete delineation between portamento and legato.4 This explanation will be confusing for us to understand if we listen to Adelina Patti’s (1843-1919) singing who was a former student of Garcia. The way she used portamento and legato were likely perceived differently by musicians of the past than by those of today. This might explain much of the pedagogical writings of the time suggesting the use of portamento with great discretion - since perhaps much of what we now hear as portamento was back then heard as legato. However, legato and portamento by function have different roles. Legato is a general rule in every kind of singing. Lamperti writes: ‘He who cannot sing Legato cannot sing well5 (chi non lega non canta). Meanwhile, portamento functions as an expressive tool. It is used to color specific words for dramatic effect, in accordance with the text.6 Musicologist Johnn Potter in his article “The Rise and Fall of Portamento” explains that portamento can also be used as a declamatory or rhetorical device for singers to show their expression.  

          Today portamento is often viewed as an unwanted effect.9 After the Second World War, portamento is one of the elements of vocal performance style that changed most radically. Potter notes: ‘taste has clearly moved on from Garcia's day, when portamento was considered an aid to sincerity.’ He argues that Lotte Lehmann reduced the use amount of portamento over the course of the war in her performances. Martha Elliot in her book “Singing in Style” suggests that portamento should not be overused as it will become a tasteless mannerism. The norms accepted by listeners in the past in this regard are very different.10 Soprano and researcher Sarah Potter thus situates portamento in today's practice as follows: 

The portamento is an element of historical singing all but ignored in modern performances of historical repertoire, despite protestations by those in the academic community. This is most likely due to its gradual decline in popularity during the twentieth century; the technique is now often regarded as old-fashioned, and something that is only to be heard used consistently in early recordings. 11 

The rejection of portamento in modern practice results in cleaner performances. This rhymes well with the ideologies of mainstream performance practice. Stam notes: 

As a result, portamento challenges the framework of neatness and tidiness considered desirable in MSPs. Teachers, juries and conductors have told me on numerous occasions not to use portamento in my performances, even in repertoires where historical evidence shows that portamento was used frequently by musicians closely associated with those works. [...] early recordings demonstrate that the technique, along with devices like tempo and rhythmic flexibility, was widely used by string players and singers connected with late-19th and early-20th century repertoires.12

          The literal meaning of messa di voce from Italian is the mass or emission of voice. It is a dynamic produced through a crescendo from pp to ff before returning with a decrescendo from ff to pp. Musicologist Richard Millers explains that this could also be a test of how well breath emission and vocal fold approximation are coordinated. Singers must understand how to maintain consistency of timbre and vibrancy during a change of intensity. He writes,  

If early depletion of the breath supply happens before completion of sustained phonation, or if vocal fold closure becomes slack before the exact moment of release, the tone is breathy and loses vibrancy. […] A singer must first be able to execute clean, vibrant onsets on short phonations to achieve proper vowel definitions and sostenuto control.17 

According to Miller, singers often misapplied messa di voce by interrupting musical phrases or poetic ideas with it, thereby destroying the stylistic effect. The constantly altering dynamics in every syllable will remove any sense of direction in the vocal line.18

          Due to the limitations of early recording technology, the horn that used as a microphone device was unable to capture the full frequency spectrum of Lehmann's voice. The strong loud sound would cause distortion or the needle jumping. The pre-war singers who dealt with the wax cylinder recordings technology, had to move back and forth to control their dynamics and remembered where they should stand at particular moments of the aria or song. Lehmann referred to this as ‘more dancing than singing’. However, despite of these limitation Lehmann succeeds to draw her dynamics line clearly. Gary Hickling the author of “Lotte Lehmann & Her Legacy” wrote “we now marvel at the ability of the artist to concentrate on singing as they moved about and dealt with the other technical considerations.”19

          What is called Tempo rubato was seen as part of this rhythmic flexibility whereas Garcia also makes use of the terms accelerando and rallentando: ‘Accelerando and rallentando movements require the voice and accompaniment to proceed in concert, whereas tempo rubato allows liberty to the voice only.’ In other words, Garcia suggests that in tempo rubato the voice will become desynchronized with the accompaniment. Garcia also argues that tempo rubato should be resorted to only in passages where the harmony is stable or only slightly varied. He described this as follows: 

By tempo rubato is meant the momentary increase of value, which is given to one or several sounds, to the detriment of the rest, while the total length of the bar remains unaltered. This distribution of notes into long and short, breaks the monotony of regular movements, and gives greater vehemence to bursts of passion.23  

Tempo rubato as described by Garcia is very different from today’s practice. Garcia’s tempo rubato might mean singing some notes longer and other notes shorter than their notated length, making them no longer synchronized with the accompaniment. This is quite different from modern conceptions of rubato, in which singers most often slow down and are followed in this slowing by the accompaniment in order to emphasize high points in a phrase or phrase-endings.  

          Lehmann’s own recordings demonstrate greater use of the kind of rhythmic flexibility described by Garcia before the war and a gradual straightening out of her rhythmic approach post-war. Her tempo rubato, as described by Garcia above appears often in her 1930s recordings while from 1941 her approach is more likely to be vertically aligned with the accompaniment. Having said this, her approach is still far more flexible than most other renowned singers of the post-war generation.