The complex number system can be obtained by taking a complex number to be an expression a + bi, where a and b are real numbers, and defining addition and multiplication in the natural way with the understanding that i2 = −1. In an extension of this idea, Hamilton introduced the following notion, originally for use in mechanics. Define a quaternion to be an expression a + bi + cj + dk, where a, b, c, and d are real numbers, and define addition and multiplication in the natural way, with
All the normal laws of algebra hold except that multiplication is not commutative. That is to say, the quaternions form a division ring. See also division algebra.