Accents in a 4-bar phrase.
An accent is the emphasizing of one tone above others by slight alteration of volume or length, or by slightly delaying the point of attack. The term is also applied to plainsong tones, i.e., very slightly inflected monotones. Accents are a form of articulation.
If the beats in any part of the music are subdivided into short beat-units, then there may be accents and "sub-accents", for example:
When the bars have 3 beats an accented note is followed by an unaccented one:
Similarly, in a 3-beat phrase the first bar will be more heavily accented than the following bars:
It's clear, then, that the "official" beat-unit of a composition is a convention, and that there are often smaller units and always larger units, both of which may be considered to be beats.