At an early age, dancers learn the first thing they must do is their barre exercises.. Pliés and tendus, ronds de jambe, frappés, ronds de jambe en l'air, and grand battements are the basics done at the bar. Starting with pliés, dancers warm up their ankle tendons and legs to prepare for the rest of the combinations. Pliés are the first movement to begin every jump for the rest of a dancer's career. Posture is important -- the back is upright with the derriere tucked underneath. Pliés at the barre are always done slowly so as to wake up the body and easily stretch the muscles and tendons. The math count is 1, 2, 3, 4, 1, 2, 3, 4, in time to the musical tempo.
Tendu, the stretching forward of the leg with a pointed toe on the ground, works the foot from a closed fifth (or first) position, sliding the foot outward but leaving the toe on the floor while extending the foot. Rond de jamb á terre has the leg making circular motions from front, around to the side, then to the back and sliding into fifth or first position again. All these are done a bit quicker than pliés. Tendu uses 1,2,1,2,1,2,1,2. Ronds de jamb can use 1,2,3, 1,2,3, 1,2,3, 1,2,3. That is a 3/4 count to the music. Waltz music reflects this math count.
All early class exercises build strength and stretch in the legs. Once moving to the center for exercises without holding on to the barre, the plié becomes a pivotal part of many exercises from this point on. Dancers might plié at the end of a routine to finish the exercise. They use a plié to transition from one step to another. Dancers also use plié to do turns and jumps. The plié is the timing factor for accomplishing turns (pirouettes) and jumps. The musical math factor tells dancers how long they have to plié, then push off, straightening through the legs and feet to a finished position in the air, then come back down in time for the next plié.
When dancers say a dancer is "off the music," they are referring to the fact that the dancer is not working the steps on the right counts of the music. As dancers, it is ingrained in bodies how to work with the music by the time most become professional dancers. Counting musical phrases sometime is also necessary when a composer uses discordant music. But even that has a math formula that can be followed to come out at the right place at the right time by the end of the music. Dancers are natural interpreters of the math of music but also, the math is in the soul of the music too.