Beer style is a loose term used to differentiate and categorize beers by various factors including appearance, flavor, ingredients, production method, history, or origin. The term "beer style" and the structuring of world beers into defined categories is largely based on the work done by writer Michael Jackson in his 1977 book The World Guide To Beer.[1] Fred Eckhardt furthered Jackson's work publishing The Essentials of Beer Style in 1989.