Tun (unit)

The tun is an old English unit of wine casks, holding about 954 litres, almost a cubic metre. Originally, it was a genuine unit of volume and measured 256 gallons, which is the base for the name of the quarter of 64 corn gallons. Its later division into 210 or 252 wine gallons was chosen to be evenly dividable by many small integers, including seven:

210 

= 2 · 3 · 5 · 7

252 

= 2² · 3² · 7