Meter and Time Signature
Basic element that keeps tones together and make them form unique musical expression is the time. A tone with a fixed onset and duration forms a smallest indivisible part in musical expression that we call a note. Timing relationship between two or more consecutively played notes determines what is called a meter (or metre) in music.
Meter is simply put a dimension on a time scale determined by the relation of timing properties of a series of notes. In Western music practice the meter developed as a series of equidistant pulses, usually called beats. It has been shown that we generate a series of equidistant pulses in our minds as soon as we are exposed to external periodic stimuli such as listening to a succession of two or more sounds. Such process is called an entrainment to the meter and is natural human reaction to listening of music.
Another automatic response is also grouping of those pulses, normally in pairs where the first pulse in the pair is always perceived as stronger. That's why for example a clock ticking is perceived as tick-tack, not tick-tick or anything else. Grouping may also be forced by accentuation to other group numbers, most notably 3 or 4.
In standard music notation time signature indicates the meter of the piece as well as the organization of it's beats within a measure. All Brazilian styles discussed here are usually transcribed using 2/4 time signature which means that a measure contains two identical metric units, each having duration of a quarter note. Smaller notes are placed to a subdivision of the metric unit down to sixteenth note level, thus two beats that are aligned to metric unit boundaries within the measure are supplemented by 6 additional beats. For the rest of the discussion the beats from 16th note subdivision will be called beats while to avoid confusion we will refer to the 1st and 5th beat as main beats.
Although measure serves as a smallest indivisible note container, from the perspective of logical grouping of notes it can be further subdivided. Notes that belong to the same subdivision are often connected with ligatures (beams). In case of 16th notes in 2/4 time signature, four of them are usually beamed that indicates they all belong to the same metric unit.