If you were to ask a modern musician what the quarter note means in Common Time the answer would be simple: “It lasts for one full beat, to be released at the beginning of the succeeding beat.” Ah, but eighteenth-century rhythm reading is not a simple “one-size-fits-all” affair. Just as spoken language has evolved over time, so has music notational language. The notation has remained much the same; it is how the notation is read that has changed. So, how is the quarter note quandary solved? Gazing at the issue through an eighteenth-century lens will answer the riddle.
Source: http://blog.oup.com/2016/10/quarter-note-eighteenth-century-rhythm/?utm_source=twitter&utm_medium=oupmusic&utm_campaign=oupmusic