Son cubano: the tres, the anticipated bass and the form everything else grew from
Son cubano explained: the tres guitar and its three doubled courses, the anticipated bass, the bongo martillo, and how the son montuno structure became the basis of salsa.
Son cubano is the Cuban form that salsa is built on. It came out of eastern Cuba and combines Spanish song and guitar practice with African derived rhythm. Its identifying instrument is the tres, a guitar with three doubled courses, and its identifying structure is the move from a sung section into a repeating montuno.
The tres
The tres is a Cuban guitar with three courses of doubled strings, tuned so that the doubling produces a bright, slightly detuned shimmer. It is not strummed like a Spanish guitar. The player picks interlocking figures called guajeos that behave rhythmically the way a piano montuno does in a later salsa band, and in fact the piano montuno is a direct descendant of the tres guajeo.
If you want to hear the connection, listen to a son recording and a salsa recording back to back and follow the tres in one and the piano in the other. They are doing the same job.
The anticipated bass
Son is where the anticipated bass line comes from. Rather than landing on the downbeat, the bass arrives on the and of beat two and holds across the bar line, leaving beat one open. Every Cuban derived dance music since has inherited this, and it is the single most important reason salsa feels like it is leaning forward.
Newcomers often hear this as the bass being out of time. It is the opposite: it is the most carefully placed part in the ensemble.
The rest of the group
A classic son sexteto or septeto is tres, guitar, bass, bongo, maracas, claves and, in the septeto, a trumpet. That trumpet part is worth noting because it is the seed of the whole salsa horn tradition: one voice, improvising short answering phrases, gradually becoming a section.
The bongo plays the martillo in the sung sections and moves to the campana bell in the montuno, another convention that carried straight into salsa.
The form
Son moves from a composed sung section, the largo, into the montuno, where a short coro alternates with an improvising lead voice over a repeating vamp. That two part structure is the architecture of salsa, of timba and of a great deal of Latin jazz arranging. Everything that happened afterwards is an elaboration of it.
The salsa guide traces what New York added to this, and the Afro-Cuban guide covers the folkloric traditions son drew on.
The Son Cubano Instrumental playlist page covers when to put this on rather than what it is.