In the early Middle Ages, time was first marked down the length of a candle in hourly increments. Next came large hour glasses, which were only good for an hour…or two.
Then, Brother Gerbert, who later became Pope Sylvester II, invented a simple mechanism in 966 that rang bells at regular intervals throughout the day to call his brethren to prayer. It was the beginning of mechanical clocks as we know them today.
image: Treaty on the geometry of Pope Sylvester II, Bavaria 12th century. Schoenberg collection.