So now am I on the hook? I signed the lease with the understanding that she would too. It's her name on the lease as well as mine. A lease is a contract right? And there are names and spaces for three signatures (mine, hers, the landlord's). If there are only two signatures, is it still valid? If so, why? By that logic, the tenant who signs the lease first is putting himself at great risk.

Afraid so. Are you saying the landlord handed you a lease he had already signed?

Thanks for the reply. Yes, he signed it and gave it to me. I signed it and then it sat on our table. And then my roommate got a new job and has to move to the west coast. I find it puzzling that an agreement with three names on it is still valid if only two sign. My logic tells me that this is an agreement between three parties. If only two sign, how is this an valid contract? Fortunately, he's trying to rent the place and accommodate both of us (as I'd like to move out if my roommate is leaving).

...I signed it and then it sat on our table....
If only two sign, how is this an valid contract? ...

If it sat on you table, awaiting your roommate to sign, and roommate never signed, the contract is not in effect until you deliver it to the landlord.

Your delivery of the writing to the landlord would constitute acceptance of the landlord's offer to rent to you.

It all hinges upon whether you delivered it to the landlord.