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The Radical Symbol

Before symbols, the words "roots" or "side" were commonly used for the square root of a number. Arab writers thought of a square number as growing out of a root, so Arabs often used the word radix, "extracting," or pulling out, the root. Latin writers thought of it as "finding" the latus, or side of a square.

Late medieval Latin writers turned radix into a single symbol Rx. This was used for more than one hundred years. The French writer Nicolas Chuquet (1484) sometimes used Rx2 for Rx, Rx3 and Rx4 for cube and fourth roots, respectively.

The symbol radical symbol was introduced by Christoff Rudolff in 1525 in his book Die Coss. It is believed this symbol was used because it resembled a small r (radix) at the time. The cube and fourth roots were as shown below:
cube root symbol fourth root symbol
Cube Root Fourth Root

Rudolff's symbol was not immediately used. The letter l (latus, "side") was often used. For example the square root of 4 was l4 and the third root of 5 was lc5. By the seventeenth century, the square root symbol was being used regularly even though there were many ways the indices were written for higher roots.