Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Variable X in Algebra
Name: Chris
Status: student	
Age:  N/A
Location: N/A
Country: N/A
Date: N/A

Why do we commonly use X for variables in algebra?


I have read different accounts of why the letter x is used to denote the unknown in algebraic equations. Renee Descartes might have been the first to do so, but why? Is it because its easier to write the letter x, or are there other reasons? Perhaps a brief review of the history of algebra will lend credence to different explanations.

Algebra has its roots in the Middle East where sciences including mathematics and astronomy flourished in the Islamic world in the 700-1450 period. Muhammad al-Khwarizmi (780­850) was one of the major mathematicians of his time and the author of a number of influential books. One of his major books is on arithmetic and another on algebra. In fact, it is his transmuted name ‘algorithm’ which we now use to refer to the step-by-step procedures for solving a problem. His algebra book is titled "Kitab al-jabr wal-muqabala" which translates to "the book of calculation by completion and reduction." The Arabic word "al-jabr" is the origin of the word "algebra" which describes the process of moving terms from one side of an algebraic equation to the other to find the value of an unknown. Incidentally, another major figure in the field of algebra is the famous Omar Khayyam (1048­1131), a mathematician and poet, who made significant contributions including describing algebraic equations whose general solutions were obtained some 400 years later.

In algebraic equations, one solves equations to obtain the value(s) of one or more unknown(s). The word for "thing" or "object" (presumably unknown thing or object) in Arabic - which was the principal language of sciences during the Islamic civilization - is "shei" which was translated into Green as xei, and shortened to x, and is considered by some to be the reason for using x. It is also noteworthy that "xenos" is the Greek word for unknown, stranger, guest, or foreigner, and that might explain the reasons Europeans used the letter x to denote the "unknown” in algebraic equations.

Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
Argonne, IL


Much of algebra developed when feathers dipped in ink were the most common writing tools. Ball-point pens, and even pencils, were not yet created. The two easiest letters to write were x and y. Mathematicians needed variables they could write quickly without mixing them up. As a result, x and y are still the two most commonly used variables in algebra. Convenience produced the tradition.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


In truth, we can use any letter we care to use in Algerbra, which is often convienient for labeling the various factors we try to calculate. For example, one might say "D=S*T", when we're trying to say that Distance traveled equals Speed times Time. In higher levels of math, and in science, there are a number of variable which are so useful, and always the same value, they are called constants. These constants often have a specific letter by which they are commenly identified to make it easy for the people using these formulas. To keep things from becoming too confusing, with everyone running around using just any letter for anything, X, Y, and sometimes Z are used for most problems.

Ryan Belscamper

Click here to return to the Mathematics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory