So far we described the construction of bossa nova rhythmic phrases solely by using additive rhythm that consisted of different numbers of 2-beat and 3-beat groups in the chord voice. With some bossa nova phrases that are borrowed from samba music there is another possible description. If you look at the following rhythm you may notice that there are interleaved sequences of onbeats and offbeats played in the chord voice:
First there are 3 onbeats followed by 4 offbeats by the end of the phrase. It might be easier to memorize the pattern by thinking about onbeat and offbeat sequences rather than 3 + 2 + ... additive scheme. It can easily be showed that all such rhythmic phrases contain exactly 2 ternary (3-beat) groups that mark the shift from onbeats to offbeats and vice-versa as well as that there are always 7 beats in such a phrase.
There are several samba phrases that use similar interleaved beat sequences and in our classification they belong to second and fourth basic bossa nova phrase groups. The differences between those phrases consist of the number of beats in each sequence and in the positions of their starting beats.
In the example from above the onbeat sequence starts with the beginning of the phrase and the offbeat sequence ends with the end of the phrase. There are also phrases where a beat sequence starts somewhere near the end of the phrase, continues with the beginning of the next phrase, then the beats shift to other (on or off) sequence somewhere in the middle of the phrase. One such phrase is popular Partido Alto samba phrase that is described in the fourth basic bossa nova phrase section.