Beat is the underlying and regularly spaced pulse of the music, measured in beats per minute. There are a fixed number of beats in a , indicated by the . Tango (2/4, 4/8, 4/4) has 2 or 4 beats per bar, vals (3/4) has 3 and milonga (2/4) has 2.
A note about “beat” compared to “count”. Musicians, say in 4/8 or 4/4 time signature, will count the beats 1-2-3-4, 1-2-3-4, etc. Dancers often misuse the word beat – at least in terms of how “beat” is defined by musicians. Whether there are 2 or 4 beats, as defined by the time signature, tango dancers usually dance to 2 primary beats per bar: beats 1 and 2 when the music is in “two”; beats 1 and 3 when the music is in “four”. “Counts” rather than “beats” is a more accurate term to use in this case. Dancers add “counts” at 2 per bar, so a 4 bar tango phrase has eight counts. That’s why the so called “Eight Count Basic” step sequence is eight counts.
(There may be a sense of 4 beats even though the time signature is 2/4. Tango very often subdivides the 2/4 beat, doubling the count from 2 to 4, effectively using a 4/8 time signature. Some tango music is explicitly written in 4/8, most are in 2/4. See Tango Time Signatures and the Beat).
A bar or measure is a small segment of music containing all the number of beats as specified by the time signature.
The two numbers, one above the other, written near the beginning of the first staff line (or whenever it changes), specifying the number of beats in a measure and the type of note that gets one beat.