A comparison of how much longer a diamond is than it is wide. It is used to analyze the outline of fancy shape diamonds only; it is never applied to round diamonds. There's really no such thing as an 'ideal' ratio; it's simply a matter of personal aesthetic preferences. For example, while many people are told that a 2 to 1 ratio is best for a marquise cut diamond, most people actually tend to prefer a ratio of around 1.80 to 1 when they actually look at marquise diamonds. And though the standard accepted range for the length-to- width ratio of a marquise diamond generally falls between 1.70 to 1 and 2.05 to 1, there are customers who insist on having 'fatter' marquises of about 1.60 to 1 and other customers who want longer, thinner marquises of 2.25 to 1.