Playing The Percentages

The percentage sign universally adopted, %, looks like the formulation of a vulgar fraction, with a tiny zero either side of a forward slash. It is easy to take it for granted, never wondering how we came to use it in the first place, let alone why the denominator is a single zero rather than 100 or even 00? After all, % as a fraction is a number that cannot be computed. The convention is followed with the per mille sign, two zeroes as the denominator, and the permyriad (per ten thousand) with three zeroes in the denominator. How did it all come about?

The concept of hundredths was well understood by the Romans, the emperor Augustus levying the centesima rerum venalium, a tax on goods sold at auction set at a hundredth of their sale price. In the Middle Ages many monetary systems used a decimal system and computations using a denominator of 100 became the norm. In the 15th and 16th centuries arithmetic texts included computations in hundredths and by the following century hundredths were commonly used to quote rates of interest.

The Italian term, per cento, “for a hundred”, was used to describe this form of fraction. Prior to 1425 all documents were written by hand, a laborious and soul-destroying task for the scribes concerned. Even the most proficient at their trade were prone to make mistakes and were eager to reduce their burden by deploying abbreviations or symbols which could be understood by the recipients of the document. For per cento, the most commonly used abbreviations were per 100, p 100, or p cento. A further refinement was to cross the shaft of the p with a diagonal or horizontal strike to denote that it was an abbreviation of the word per.

The next refinement, evidenced in a manuscript dating from around 1425 to 1435, was to adopt the abbreviation “pc” with a tiny loop or circle to depict the ending -o used in Italian numeration. Two and a half centuries later, an Italian manuscript dating from 1684 shows that the “p” had become little more than a squiggle and that the “c” had changed into a closed circle with a short horizontal stroke above it. Sitting above the horizontal stroke was another closed circle.

It was not until the 19th century that further changes were made to develop the symbol that we know today. The squiggly “p” was dropped and the horizontal stroke was set at an angle with the two zeroes positioned either side of it, one higher on the page than the other. We have never looked back since.

Clearly, though, the genesis of the sign came from the adoption of a commonly understood and accepted abbreviation for “per centum” than the desire to represent the number as a fraction of a hundred, hence the single zeroes. It makes sense when you know!

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