The onset of the note, or it's absence, known as a rest, at a given time constitutes a musical event. When a series of notes and rests are played they may be ordered in different combinations. The order in a combination of successive musical events is called a rhythm. Thus the unique combination of notes and rests played in a succession will determine the rhythm of the musical piece.
It's important to note that there is no repeating or recurring condition in the definition of rhythm. A rhythm exits even if there are no repeating patterns of notes and rests and rhythm may continuously change during the course of music. However in reality we will usually be able to identify some rhythmic combinations that are occasionally repeated. We'll call such a combination a rhythmic pattern and for the sake of simplicity we will limit it to a single metric unit consisting of 4 beats.
The order in succession of rhythmic patterns discussed above is a characteristic of particular music style. Similar to definition of rhythmic pattern we will define a rhythmic phrase as unique combination of rhythmic patterns played in succession. Just like rhythm and rhythmic patterns, the phrases may change over the course of music and they may repeat immediately after being finished or at some later time or even not at all.
Important characteristic of Brazilian music styles, particularly bossa nova, is that during the song the rhythm is continuously changing. It can be thought of as a rhythm being composed just like melody and harmony are. Therefore we will typically find many different rhythmic phrases within a single bossa nova song.
Being a polyphonic instrument a guitar often provides melody, harmony and rhythm at the same time. We relate to these components by using term voice. Typically three voices are found in musical scores, upper provides melody, lower bass, while middle fills in the additional notes that together with other voices form chords.
Note that term voice is not used in the strict sense as that would imply only one tone per voice at any point in time. As we use four or more right hand fingers in guitar playing we can therefore play more notes simultaneously. We group all notes that don't belong to bass or melody to a single voice that we call chord voice.
Different voices on guitar typically play rhythmic phrases of different percussion instruments found in Brazilian rhythms. Usually bass voice plays rhythm of surdo drums, while chord voice plays rhythm parts of tamborim, pandeiro and agogo.
Imagine there are two voices being played at the same time as shown on staff line below. Upper voice plays additive rhythm in the form of 3 + 3 + 2 + 2 + 3 + 3 containing 16 beats in the phrase, while lower voice plays simple divisive rhythm with 4 played beats in the phrase. In order to match those two rhythms we can write lower rhythm in 2/4 time signature where two measures contain 16 beats of upper rhyrhm on 16th note subdivision level.
Bottom rhythmic phrase shows how combination of different concurrent rhythms produce what is known as a composite rhythm or polyrhythm. We will show construction of Brazilian rhythms based on composition of additive rhythmic phrases consisting of a combination of one or more binary (2) and one or more ternary (3) beat groups, each additive phrase having length of 8 or 16 beats, with a simple divisive 2/4 rhythm, just like in the example above.