The "Ideal Cut" is a cut based on a specific set of proportions for a round brilliant diamond proposed by gem cutter Marcel Tolkowsky in 1919.  While Tolkowsky's original theories presented only one particular combination of proportions for creating the best balance of brilliance and dispersion, today the American Gemological Society recognizes any diamond falling within a narrow range of proportions and finish quality as being an "Ideal Cut" (also called an "AGS 0" or "AGS triple zero").