Imperial stouts stout Imperial Russia dubbel Trappist beers tripel session India pale ales abv beers ales
Additional markers are applied across styles.  The terms "imperial" or "double" are used interchangeably for a higher-alcohol version of a particular style.  Originally applied to Imperial stouts, a high-alcohol style of stout brewed in England for export to Imperial Russia, the term imperial can now be applied to any style name to indicate a higher alcohol content.  "Double", meaning the same thing, originated with the dubbel style of Trappist beers in the 19th century.  Even higher alcohol-content beers can be labeled "triple" (from the Trappist tripel style) or even "quad".  Lower-than-standard alcohol content is often indicated by the term "session".  For example, while India pale ales often have alcohol content around 6–7% abv, a "session India pale ale" will often have alcohol content below 5%.  
