The buttery
The buttery is where the drink was kept: ale, beer, cider and mead, stored in cask and drawn as needed. Its contents were the butler's responsibility, and the word itself has nothing to do with butter: it comes from the butts the liquor stood in.
In the standard plan of a great house of this date the buttery sits at the lower end of the hall, paired with the pantry across a short passage, with the kitchen beyond. Drink came from one side and bread from the other, and both were carried the same short distance to the tables. The arrangement is legible here because the rooms were never knocked together.
A butler in this period was not a figure in livery answering a door. He was the officer responsible for a substantial and valuable store, accountable for what came in and what was drawn, and the position sat near the top of the household's below-stairs hierarchy.
Related pages: the south door of the great hall.
Pantry and buttery are not the same room
The two are constantly confused and the distinction is simple. The pantry held bread and dry goods, from the French paneterie; the buttery held liquor in butts and casks, from bouteillerie. Neither has anything to do with butter. They sit side by side at the lower end of the hall, either side of the screens passage, and each had its own officer.
The arrangement survives in the modern word for a college buttery and in the layout of a great many old houses, where two doors of equal size flank a third leading to the kitchen. Where all three survive, as here, the household plan of a late-medieval establishment can be read directly off the building.
What was actually stored
Ale before beer, and both in quantity. Ale brewed without hops kept only a few days, so a household of any size brewed continuously and the buttery worked as a short-term store rather than a cellar. Hopped beer, which arrived in England during the fifteenth century and was well established by the time this building went up, kept far longer and changed the economics of the room considerably.
Cider and mead appear in household accounts of the period alongside them, and wine where the family could afford it, though wine was usually kept separately and under closer control. Consumption figures in surviving accounts look extraordinary by modern standards. They are less so once you remember that ale was weak, was drunk by everybody including children, and was the only reliably safe drink available.
Questions about the buttery
Why is it called a buttery if there is no butter?
The word comes from butts, the casks the drink was kept in, by way of Old French. The resemblance to butter is a coincidence of English spelling.
What did a medieval butler do?
He was accountable for the drink store: what came in, what was drawn and what was lost. It was a position of trust near the top of the household's service hierarchy, not a door-answering role.
How much ale did a household drink?
Household accounts of the period routinely record a gallon a day per adult. The ale was weak and served as everyday drink and not as an indulgence.
Why do so few butteries survive?
Because service rooms were the first thing later owners rebuilt. A house that still has its pantry and buttery is usually one that was never comprehensively modernised.