Editing Liber Abaci (section)

== ''Modus Indorum'' ==
In the {{Lang|la|Liber Abaci}}, Fibonacci says the following introducing the affirmative ''Modus Indorum'' (the method of the Indians), today known as [[Hindu–Arabic numeral system]] or base-10 positional notation. It also introduced digits that greatly resembled the modern [[Arabic numerals]].

{{Quotation|As my father was a public official away from our homeland in the [[Béjaïa|Bugia]] customshouse established for the Pisan merchants who frequently gathered there, he had me in my youth brought to him, looking to find for me a useful and comfortable future; there he wanted me to be in the study of mathematics and to be taught for some days. There from a marvelous instruction in the art of the nine Indian figures, the introduction and knowledge of the art pleased me so much above all else, and I learnt from them, whoever was learned in it, from nearby Egypt, Syria, Greece, Sicily and Provence, and their various methods, to which locations of business I travelled considerably afterwards for much study, and I learnt from the assembled disputations. But this, on the whole, the algorithm and even the Pythagorean arcs, I still reckoned almost an error compared to the Indian method. Therefore strictly embracing the Indian method, and attentive to the study of it, from mine own sense adding some, and some more still from the subtle Euclidean geometric art, applying the sum that I was able to perceive to this book, I worked to put it together in xv distinct chapters, showing certain proof for almost everything that I put in, so that further, this method perfected above the rest, this science is instructed to the eager, and to the Italian people above all others, who up to now are found without a minimum. If, by chance, something less or more proper or necessary I omitted, your indulgence for me is entreated, as there is no one who is without fault, and in all things is altogether circumspect.{{r|devlin-ff}}}}

{{Poem quote|The nine Indian figures are: 
9 8 7 6 5 4 3 2 1 
With these nine figures, and with the sign 0 which the Arabs call zephir any number whatsoever is written...{{r|9-figures}}}}

In other words, in his book he advocated the use of the digits 0–9, and of [[place value]]. Until this time Europe used Roman numerals, making modern mathematics almost impossible.  The book thus made an important contribution to the spread of decimal numerals. The spread of the Hindu-Arabic system, however, as Ore writes, was "long-drawn-out", taking [[History of the Hindu–Arabic numeral system#Adoption in Europe|many more centuries]] to spread widely, and did not become complete until the later part of the 16th century, accelerating dramatically only in the 1500s with the advent of printing.{{r|ore}}