I Guess,

we have all seen old metal banded woodened water butts, 

photograph Celtic Water, that look like this, (this is a new one from Celtic Water), but you get the idea, but what many will not know that a butt is actually a measure of volume, back in my youth if someone was relating how much they had to drink the night before, "I had a 'buttfull'", was a commonly used expression, so how much is a buttfull? never underestimate the power of the Internet, 

illustration taken from Names and contents of beer and ale vessels in James Lightbody's, Every Man His Own Gauger, 1695, found on Wikipedia, as it happens looking at one of the measures above, every time I visit Duncan,

 I go past the Fox & Firkin just outside of Lewisham, and way back in 2012, 

we saw The Firkin Pub in the Philippines, and although I do not have a photograph of it, in Beckenham there once was a pub called The Three Tuns, a regular watering hole in my misspent youth!