Relative density (also known as specific gravity) is a measure of the density of a material. It is dimensionless, equal to the density of the material divided by the density of water. When layering drinks, a bartender should have a pretty good idea what liquids are heavier than others. No amount of technique will enable Kahlua to float on water (why would you want to?).

The chart below will help you determine how certain liquors will stack up against each other. If you don't know the density of a liquor and can't compare it to one on the list, keep in mind that lower proof often means higher relative density as the extra sugars weigh the liquid down.

A Tequila Sunrise is a good example of layering tequila, grenadine, and orange juice.