Specific question about Bid Day

[ILuvGreek]

I read all the relevant stickies and tried to do a search for "bid day" with no success in answering my question. I apologize if my question is out there, but I've had no luck in finding a specific answer despite all the great stickies.

Let's say you are invited back to up to 3 houses for Pref Night. After Pref Night, do you indicate in order which houses you are interested in or only the one house you would like to pledge?

Do they then match up for Bid Day according to your preference and the houses' preferences? Let's say I want to pledge house C, but list them in pref order as house C, house A, and house B. This is the point where I get confused. If my #1 choice doesn't want me, but my last choice does, does my bid card come from my last choice? Of course it's always possible there will be no matches on Bid Day. Are the bids computer cross-matched?

Can someone please clarify the very last step in more detail. Thanks!