In our research and development lab (as I consider it) at CCPA , Roosevelt University where I teach, I am always trying to develop new answers to old questions, many times in response to specific problems my students are having. We came upon one awhile ago when working on that most basic of concepts, the marcato/tenuto style, which I believe accounts for about 75% of what we play. The problem most people have is, if playing articulated quarter notes tenuto, there will be a space between notes. If asked to play with no space between notes a pseudo-legato style will result. I am a firm believer in the idea that when a composer writes a slur or non-slur indication in a passage, they expect some attempt to differentiate the two and hopefully make each marking audible to an audience.
A good way to sort out and develop skill in the different styles of articulation is to start with what I consider a neutral kind of articulation. One that is neither slurred nor articulated. This is accomplished by playing a series of notes with a D and T syllable combined so that neither is dominant. As featureless as this is, there is a place for it in the arsenal of musical tools. It fits precisely the style marking when both dots and slurs are placed over a phrase, something which has been misunderstood by many players. In my experience almost everyone has interpreted this marking as mostly staccato and very little legato, whereas a combination of both gives a unique style of a soft articulation coupled with continuous flow of sound, of which an excellent example is the chorale from Brahms 1st symphony.
Using the neutral half tongue-half slur as a model, it is then time to change the syllable from equal parts D and T to more T and less D, although the remaining D syllable will allow the sound to be continuous right up to the next note, a crucial feature of the marcato/tenuto style. The half tongue-half slur however, should not be used in place of either a slurred passage or a tongued passage except in a few specific excerpts such as the louder sections of the Mahler 3rd solo or the solo passages from Sibelius’ 7th symphony. Using an even, indistinct articulation on everything only narrows the options available to a player, and makes the trombone a one dimensional instrument, something which the trombone world should be trying to overcome. Additionally, an important point to remember: an articulated note in the middle of a phrase has every right to be as clear as the first note of that phrase. The widespread habit of only getting clarity on the first note and the rest of the notes in a phrase emerging muddled because of a desire for tenuto, is something that can be overcome with thought and practice.
The remaining concept of a true legato can be derived from the neutral half tongue-half slur by reversing the T and D, and putting D to the forefront when playing so-called legato slurs. Legato tongue should not be used in natural slurs, as it is unnecessary, and will only intrude on the smoothness of those slurs. The amount of legato tongue necessary for legato slurs is dependent on the register they are played in. Generally speaking, the lower the register, the more legato tongue is necessary to avoid smears. There is a worldwide epidemic of reluctance to use enough legato tongue in legato slurs, thereby making it necessary to either move the slide in a jerking manner, or dropping the air in-between, both of which eliminate the possibility of a true legato, something of which the trombone has an infinite capacity for. If Martin Luther said; “why should the devil have all the good tunes,” then I say: why should the jazzers have all the good slurs? The advantageous reason for using legato tongue in legato slurs is the amount of variety possible, so that every register can have the perfect amount of legato tongue necessary to produce the same liquid smooth slur from top to bottom.
To sum up; using the neutral half tongue-half slur model as a starting point; (D and T syllables in more or less equal strength,) to produce a clear articulation with no space between tenuto notes, the T syllable takes the place of the D. Important: it is necessary to completely seal the embouchure for a fraction of a second to produce a clear articulation in the middle of a phrase, something which many players are reluctant to do. It is possible to achieve this with virtually no space before the next note if the airstream is able to instantly replace the T syllable. Conversely, using the neutral half tongue-half slur as a starting point for developing a true legato, the T syllable retreats and the D syllable takes the dominant role, allowing the player to keep the sound flowing between notes, the secret in gaining more resonance in legato playing. The neutral articulation starting point shows us that it is not clear enough for an articulated passage, and not smooth enough for a slurred passage, and that unsuitability for both basic playing modes can be used as an illustration that both need to be modified in opposite directions to enlarge our repertory of expressive tools.