In 1202, Fibonacci (a.k.a. Leonardo of Pisa) published Liber abaci ("book of the abacus"), the first European books to use decimal notation in arithmetic (which Fibonacci learned in his native North Africa). The spread of decimal arithmetic made possible the great "Age of Navigation" and "Age of Trading", which followed -- although historians choose to ignore this.

Can you imagine trying to balance your bankbook or fill your tax statement with Roman numerals? In Fibonacci's time, the University of Bologna was the only place in the world which taught the best methods for dividing with Roman numerals.

                

In 980 Gerbert (later crowned as Pope Sylvester II) introduced gobar decimal numerals on the abacus, in vain attempt to replace Roman numeration by decimal numeration.

Many merchants and others discovered with writings of decimals were regarded as possible Islamic spies or traitors to to the State, and by the Church as heretics. This led to punishment, often unto death.