No, it's not a Yorkshire square

It's a tradition, or an old charter, or something that wheneven someone who knows a bit about beer sees an open square fermenter they ask "is that a Yorkshire square?". The answer is nearly always, no. 

Open fermenters are the exception not the rule nowadays so it's usually only in old breweries that you see them. The fact that a fermenter is open and not round does not make it a Yorkshire square though. Actual Yorkshire squares are considerably stranger. If you can see a fish tail it might be a Yorkshire square, but only if the vessel is divided by a deck too. 

Diagram of a Yorkshire square fermenter

Cross sectional diagram of a Yorkshire square

They were originally made of stone, and the ones I saw at Sam Smith's were made of slate. They have been made of stainless steel though, which I think they had at Tetley's, and there are stainless round ones (Yorkshire rounds!) at Black Sheep. 

They have a lower level separated from the upper level by a deck. The deck has pipes known as organ pipes running down to the lower level, and a large opening with a flange around it. They were mainly, but not exclusively, used in the north of England. I guess the clue's in the name with that one!

An empty Yorkshire square showing the upper deck

They were developed to work with highly flocculent ale yeasts, i.e. yeasts that rapidly clump together and rise to the top of the fermenter. 

Yorkshire squares being roused at Sam Smith's

The lower compartment of Yorkshire squares is filled with wort and yeast, and so is an inch deep layer on the upper deck. During fermentation yeast rises through the large opening and beer drains back into the lower compartment through the organ pipes. 

Yorkshire square in use

Rousing is used to keep the highly flocculent yeast in contact with the beer - the beer is recirculated by pumping it through the fish tail on to the upper deck. At the end of fermentation the yeast can be skimmed from the deck. 

As well as being a way of fermenting with, and collecting, highly flocculent yeast Yorkshire squares are also said to make beers with a fuller palate than beers fermented more conventionally. 

I'll end on a video one being roused: