Euclid described it best in the Elements as a balance of proportion between two lines:

“A straight line is said to have been cut in extreme and mean ratio when, as the whole line is to the

greater segment, so is the greater to the less.” INTERFERENCE - A Grand Scientific Musical Theory, Richard Merrick, 2011, Third Edition

See Also

**Golden Mean**
**Golden Number**
**PHI**
**Proportion**
**Propositions of Geometry**
**Ratio**