What makes music danceable?

Recently... I was already about to go to sleep, when I started thinking about his question: What makes a Tango danceable?

As a Tango-DJ, this is the most relevant issue, when choosing music for a Milonga. Of course, the matter of danceability depends on the general proficiency level of the dancers at this specific event. A beginner will need a „simpler“ music, than a fully trained, professional stage dancer and artist. 

But let‘s evoke a normal local Milonga in a bigger town. It‘s a social event, so you‘ll find none or only very few stage dancers, a few (mostly semi-professional) teachers, some fairly advanced dancers, a majority of intermediate people and a bunch of beginners. That‘s what you have to deal with.

Your choice of music thus has to be „simple“ enough to be danced by the beginners and intermediates, but needs to appeal to the more advanced dancers as well. So changing the level of „simplicity“ within the course of the Milonga or alternating between different levels is the sensible thing to do.

But lets look at the defining factors. In my opinion, a danceable Tango (Vals, Milonga) is characterised by:

A perceivable beat:

An instrument (often the bass or piano) plays the beats within the measure (1234 or 123 or 12) and you would be able to count them to go along. Some Tangos by Biagi (e.g. Belgica) may be quite hard to dance to, as he makes pauses and lets drop notes and beats completely within the measure. In some very lyrical dances the legato passages of the violins and bandoneons may be so dominant, that you will not hear the beat for a longer time. An experienced dancer will have no problem with these cases, as he's going to feel the beat virtually through the strong notes of the melodic parts. But for a beginner, these Tangos might pose a problem. 

A constant speed:

The speed does not change dramatically during the course of the Tango (Milonga or Vals). A typically slower „introduction“ or finale to e.g. a romantic Vals is no problem, but most of the time, the speed should be constant. Lots of Tangos by Pugliese change speed quite often, and are thus not easy to dance to.

An acceptable speed:

The speed should be in a danceable range. If a song is too slow (e.g. modern music by Los Cosos de a Lao) or too fast (some d‘Arienzo Valses or Milongas) it might get too challenging, depending on the quality of the dancer's technique. People are either lacking the balance or the the stamina to keep up with such a speed.

An acceptable sound quality:

Let‘s face it, some of the very old Tangos might be very sweet and even easy to dance to, but the sound quality is just not acceptable. If all you hear are scratches or noise a Tango is not danceable any more.

Manageable rhythmic variation:

Tangos do not have a constant basic rhythm like Milongas (Habanera rhythm) do. The melody may form new rhythmic patterns in every measure and additional rhythms will be played in overlaid melodies or as an accompaniment. That makes a Tango in general much harder to dance to, like a - let's say - Rumba or Milonga. In order to not simply step on the beat (the minimum requirement), the dancer actually has to know (or be able to guess well) the melody. 
As long as the rhythmic patterns are very common ones (a simple quick-quick-slows like 123 or a slow-quick-quick like 134), this is not a big deal. These rhythms only use notes that are already represented by the basic beat, so even if you dance the other variation, you will not be really wrong. Syncopations make it already harder, as you have to know when they will be played. You should not dance a syncopation, if the orchestra does not play it. Uncommon rhythms like 3-3-2 or Triplets make it more challenging - even for very advanced dancers. As a DJ, I often sit and watch the dancers move to the music and check, if the get the rhythmic variations. Even at events with a very high dance level, only few people use them. That's of course no catastrophe, but it's still sad.
By the way: lots of rhythmic variations in Tango music are based on the Habanera (1+34) pattern by adding or leaving out notes. Once you discover this fact, you'll find it much easier to dance to them.

An acceptable complexity:

There are simple Tangos and complex ones. One way to measure this factor depends on the number of layers within the music. A Tango is like a cake: A simple one will consist of a layer of basic beat (the bottom) and one „hummable“ melody (the cream filling). This melody will most likely alternate with a second melody. If there is a third melody or two of them are overlaid, it can get already quite complex. Think of a lovely piece of "Schwarzwälder Kirschtorte". Tastes super, but try to bake it! ;-)
Check out "Nueve Puntos" by Di Sarli: It is played in hundreds of beginners classes, but if you examine it closely, you'll discover that it is a quite complex piece of music. I don't say that it is not danceable, but it is certainly no "simple" Tango.
A singer as a part of the orchestra can take over the melody during a shorter period of the Tango. This will not necessarily interfere with the danceability, as long as he does not add a variational phrasing.

A predictable course:

The predictability depends on the overall composition of the Tango. If there are distinguishable parts within the music, and not more than 3 different melodies are repeated after a Rondo-structure or variations thereof (e.g. ABABA or ABABC), a dancer can guess easily, which part of the melody will be played when and can adapt his movements to the flow of the music. A more surprising order of parts (ABABBBB in "Adios para siempre" by D'Agostino) makes it already more challenging to dance to, when you hear it for the first time. But if the melody is almost totally free (no repetition of larger melodic parts like in "El Yaguaron" by Biagi), forget about it. 
Also check out the Tango's phrasing. A rather "predictable" Tango will have regular phrases within one part: typically 4 measures form one phrase and 4 phrases one part (4/4/4/4). An "unpredictable" Tango may be using irregular phrasing patterns - e.g. 3/3/4/3/3 in part B of Humillacion by Biagi.

An emotional appeal:

Music speaks to you. It will evoke emotions, inspire you. This makes the difference  in between just moving to the beat and dancing, especially if you are a more advanced dancer. This factor is of course very subjective, because here we touch the question of how much we „like“ a Tango or not. But nevertheless, it is crucial. I will not dance to a Tango, if I don‘t like it and - here it gets even more subjective - in my opinion this depends on the „humanity“ of the music. A perfectly composed Tango by De Caro may be „complex, challenging and interesting“, but it may not be as emotional as a Tango by Rodriguez with the singer Armando Moreno. So very often, it is the human voice and even the content of the lyrics, that creates the „humane appeal“. Sometimes, it may be also an humane attribution of a specific instrument - the wailing violin of Elvino Vardaro for example. In Buenos Aires Milongas, you‘ll hear the old Milongueros sing along to their favourite Tangos and many DJ's will cater to that taste.

So a good DJ will have a feeling for which Tangos make the dancers sad, happy or emotional and will even know favourites of special dancers. This last factor is it what distinguishes a danceable Milonga from a great one.

A note:

The more varied the rhythmic variation of a Tango is, the more layers you'll discover in it, the more "unpredictable" it gets… the more interesting a Tango may become to musical or advanced dancers. That's for sure! But in this article, I am focussing on the "danceability" for average dancers. 
So, dear DJs: please add the more complex Tangos to your repertoire, but keep in mind, that you have to adapt the level of complexity to the dance-level of your dancers. Challenges are fine, but you don't want the audience to fight with the music. 

(I revised this article on October, 22, 2016)