In the 19th century, ale brewing styles shook out into four broad categories based on the excise taxes applied to alcohol. While originally various types of beer were brewed in each category, eventually the different categories each became associated with a particular brew. In the 1970s, almost a century later, these categories were revived by the new "real ale" movement (the UK's equivalent to the microbrewing craze). So today, people still refer to beers by the tax (in shillings) a hogshead of said beer would have incurred in 1885.
The main categories are as follows:
- 60/- (aka "Light") under 3.5% ABV
- 70/- (aka "Heavy") 3.5%-4.5% ABV
- 80/- (aka "Heavy" or "Export") 4.0%-5.5% ABV
- 90/- (aka "Wee Heavy") over 5.5% ABV
The other appellations have fallen out of use for a variety of reasons. Cheap lagers have pushed out the 60/- style, for example, and English, Irish, and American varieties (pale ales, bitters, porters, stouts, etc) have encroached upon the scene. But the 80/- every bit as Scottish as haggis and whisky--Caledonian in Edinburgh and Williams Bros. in Alloa make the best examples, if you ask me.