A measure, often called a bar, is a small section of music containing all the number of s as specified by the . In printed music each measure is contained within two bar lines – a vertical line across the five horizontal lines (called staff lines) – one at the start, the other after all beats have been accounted for, at the end.
Three bars in 2/4:
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 bar, indicated by the time signature. 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.
(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).
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.