After I learnt to distinguish sentences from periods, I began to wonder, by which factors it is determined whether a musical phrase grows to become a period or a sentence. My hypothesis is that it depends on whether a figure begins similarly to the previous, or if it ends similarly. The former case logically grows into a sentence, the latter into a period.
The way I see it, a sentence is born when the order of the last middle two phrases in a period are swapped. Is this incorrect?