There is a letter that is the protagonist of any **equation**function, proportion: the * x. *Anyone who has ever performed one of these calculations has encountered it. But why was this letter chosen to indicate “

**the unknown**“?

Its history is almost adventurous: it was born from

**Arabic pronunciation**of the word

**“What”**but it could find an affirmation in modern mathematics thanks to a funny and casual anecdote concerning the French philosopher and mathematician

**Descartes**.

The history of mathematics and its language is closely linked to the **Arab culture**. Our numbers, in fact, although having their origins in India around the 4th century BC, then arrived in Western culture through Arab scholars such as al-Khuwarizmi and his work *Of the Golden Numbers. *Mathematicians of al-Khuwarizmi’s time referred to what we call “**unknowns**” of equations like **“the thing”**meaning “what was to be discovered”. In Arabic, “what” was said ** shay**whose

**pronunciation was very similar**to the pronunciation of our

**letter**

**x**.However, when these notions arrived in Europe, the use of the term was not immediate. *x* within equations and in the Renaissance period the concept of “thing” continued to be used. However, the Arabic pronunciation was not forgotten and reached the ears of the great philosopher and mathematician **Rene Descartes** who, in his book * Geometry *In the

**1637**consecrated the use of both the term

**in reference to “that of which we do not know the value”, that the**

*unknown***letter**as its symbol.

*x*Descartes’ fame has reached our days both for his mathematical merits, think of the Cartesian axes, and for the very famous philosophical phrase «** I think therefore I am»**. It is perhaps from the union of these two that another important intuition of Descartes is born: to define

**what we – literally – don’t know within an equation.**

*unknown*When in 1637 he is writing the treatise *geometry, *Descartes decided to use the **Initial letters** of the alphabet (* a, b, c, …)* to indicate the

**known quantities**while those

**finals**(

**) as a symbol of the**

*z, y, x, …***unknowns**Logically, it would have been correct to use the first letter for the unknown in an equation as

*z,*being the last letter of the alphabet, and continue backwards with

*y, x, …*for the subsequent unknowns. The

**more frequent use of the**

*to the detriment of*

**x***z*And

*y*It is generally attributed – at least this is the most widespread hypothesis – to the fact that Descartes

**knew**the history of the term “thing” and its

**Arabic pronunciation**so he opted for the letter

*x*so close in pronunciation to Arabic

*shay*.

There is a**another hypothesis**certainly more adventurous. It is said that the printer in charge of printing *Geometry* was short of *z*and since the ** x era** and it’s a lot

**less frequent in French**proposed to use the latter and Descartes accepted – perhaps always mindful of the Arab heritage.

There **letter ***x*has then acquired fame over time as well **outside the mathematical world**always to indicate **something whose nature is unknown**. An example is the **X-ray**called this way precisely because initially it was not known what they were. *x *It has also been used in films or TV series to indicate, for example, superheroes with unknown powers (the **X-Men**) or investigations to be kept secret (the **X-Files**)*. *