UK's oldest registered trademark is the red triangle of which brewery?

The Bass Brewery was founded in 1777 by William Bass in Burton-upon-Trent, England. The main brand was Bass Pale Ale, once the highest-selling beer in the UK. By 1877, Bass had become the largest brewery in the world, with an annual output of one million barrels. Its pale ale was exported throughout the British Empire, and the company's distinctive red triangle became the UK's first registered trademark.

Bass took control of a number of other large breweries in the early 20th century, and in the 1960s merged with Charrington United Breweries to become the largest UK brewing company, Bass Charrington. The brewing operations of the company were bought by Interbrew (now Anheuser-Busch InBev) in 2000.

Draught Bass (4.4% ABV) has been brewed under contract in Burton by Marston's for AB-InBev since 2005. Bottled and keg products are brewed at AB-InBev's own brewery in Samlesbury for export, except in the United States and Belgium, where Bass is brewed locally. Bass Ale is a top ten premium canned ale in the UK, with 16,080 hectolitres sold in 2010.