<ref name=citeboyer>{{HarvHarvnb|Boyer|1991|loc="Europe in the Middle Ages" p. 258}} "In the arithmetical theorems in Euclid's ''Elements'' VII-IX, numbers had been represented by line segments to which letters had been attached, and the geometric proofs in al-Khwarizmi's ''Algebra'' made use of lettered diagrams; but all coefficients in the equations used in the ''Algebra'' are specific numbers, whether represented by numerals or written out in words. The idea of generality is implied in al-Khwarizmi's exposition, but he had no scheme for expressing algebraically the general propositions that are so readily available in geometry."</ref>
