Lacing refers to the foam that clings to the sides of a glass of beer. Beer enthusiasts often associate a brew’s quality to how much of the web-like foam is left after you’ve finished your drink. The more lacing, the better the brew. If you find yourself staring at an empty glass void of the desired beer lattice, it’s probably due to a lack of care taken during the brewing process or the use of low-quality ingredients. That, or you just have a dirty glass. (An oily glass will prevent the foam from sticking to the sides.)
Note: Some beers like lambics, are excused from this rule due to low head retention.
(photo via the beer czar)