House Yeast. What does it mean?

When a brewery says they use a "house" or "proprietary" strain of yeast, what exactly does that mean? Do they get custom "blends" from White Labs or Wyeast different from what is available to homebrewers? I assume it is not just how a yeast changes over generations of use, since I think most breweries go back to a pure strain after some number of batches. Anyway, just curious.

my understanding is that they use one type of yeast for all (or most) of their brews, often it is a strain that is available to us homebrewers. Many commercial breweries get their yeast from white labs or wyeast as well.

"house" just means the yeast they use, it might be from white labs, wyeast, wehenstephan, fermentis etc or it might not. Proprietary means it is their intellectual property. Pacman is proprietary but Wyeast maintains the culture as a service to Rogue and Rogue allows them to market it to homebrewers. Some breweries, generally large ones, maintain their own culture (maybe with a backup at a local hospital in cryogenic storage with redundant power back up).

A lot of breweries will tell you what yeast they use if it is a stock strain from a vendor. If it isn't they should at least tell you if you can culture it from a bottle (ie it is bottled with the primary strain, not pasteurized and not sterile filtered). Some breweries, usually small ones or pubs, will give you a growler full of yeast if you ask nice and swing buy while they are brewing.

How are the proprietary strains developed? For example, how did Rogue happen upon Pacman? Just trying to get an idea of how these proprietary strains are developed.

Interesting to note the marketing of using the phrase "house yeast"--it may be the exact same yeast we are dumping in our beers, but it sounds special.