Final answer:
A song that begins on the last beat of a measure features a syncopation, which is different from polyrhythm (multiple contrasting rhythms) and offbeat (notes between the main beats). Syncopation requires skill to perform as it emphasizes normally unaccented beats, thus adding complexity.
Step-by-step explanation:
When a song begins on the last beat of a measure, it is said to begin with a syncopation. Syncopation occurs when a rhythm emphasizes a beat that is usually not accented, creating a counterpoint to the regular rhythm. This is different from polyrhythm, which involves multiple contrasting rhythms being played simultaneously, and offbeat, which refers to notes played in between the main beats of the measure.
For example, in the phrase "There was an old woman who lived in a shoe," if the word 'old' were accented and began on the last beat of a measure before the regular downbeat, it would create a syncopated rhythm. Syncopation adds a layer of complexity and interest to the music, necessitating skill from the musicians to execute the off-the-beat pattern smoothly.