As metric units are being divided by two we get twice the number of beats in a subdivision. As already mentioned every pair of beats is perceived as strong-weak. The difference in the perception of the beats comes solely from their timing, as indicated by their placement within the measure.
With every subsequent division of metric unit we get new pair of beats where odd beats are aligned to the beats of the previous subdivision. On the 16th note level we get 2 strong beats that are aligned to previous division levels and 2 unaligned weak beats in a single metric unit.
In samba and bossa nova all main beats are normally played. In samba music accent is sometimes placed on second main beat of the measure, more on particular ways of accentuation will be discussed later.
It is often being said that syncopation is an important feature of Brazilian music. It is established by delivering a musical event at unexpected time. Such an event affects meter perception and induces attention in the listener's mind. This is achieved by playing the note on even (weak) beat and sustaining the same note or placing the rest on the following odd (strong) beat. This is known as a syncope. Syncope may be continued by playing the following weak beat(s) or finished by playing the following strong beat. This can bee seen also as moving the note from strong beat to the preceding weak beat. Some important facts without going too deep into theory:
1. Syncope can have different strength depending on context (patterns) as well as various other factors, such as the relative strength of the syncopated beats, the accentuation of syncopated note(s), etc.
2. It is possible both to enforce and diminish syncopation effect using various techniques.
3. The perception of meter may be altered using other means as well.
This figure shows how syncope is built from 3 notes played initially at expected moments and after moving the 2nd note it becomes syncopated. Two versions are showed, one with the rest on strong beat, the other with sustained syncopated note.