Algebra was first described by the Persian mathematician Muhammad ibn Musa al-Khwarizmi around 830 CE. He called it Al-Jabr, a term that can mean “completion” or “balancing.” In his important book, al-Khwarizmi explained how to work with unknown quantities. He used the Arabic term ash-shai’, meaning “the unknown thing,” for an unknown variable. The word algorithm also comes from the Latinized form of his name. Algebra later spread from the Islamic world to Europe, with Spain playing an important role. Spanish does not have the “sh” sound found in Arabic, so some scholars used the Greek letter chi (χ) to represent this sound when translating mathematical ideas. Here is what uppercase and lowercase Chi look like: