Exceptions to the prosodic rules

 Suppose there is an exception to the rule that strong metrical beats will fall on beats 1 or 3.  

For example, I found in a Taylor Swift song a phrase in which there is a strong beat on 1 2 3 4 of a single measure. Or you could anticipate a beat, singing it on the and of 4 of the previous measure, or delay an emphasis to later in the measure.  

A subsidiary of this rule would be that breaking it involves syncopation or a special kind of phrasing.  The breaking of the rule is felt to be different. In practice, where the accent fall is more fluid, but that does not prevent 1 and 3 from being the strong beats in principle.  

I'm not quite sure what to do with the practice of certain vernaculars in which the 2 and 4 receive emphasis.  For example, you don't clap your hand of snap your fingers on 1 and 3 in gospel or jazz. A jazz player will set a metronome, but treat that as a hi hat, on 2 and 4, not as a downbeat.  

 I would say that they are still the 2 and 4: otherwise they would just be the 1 and 3!