Left side, always. No idea how that was decided 43 years ago when we got married. Probably should be the right side in the summer as my wife likes the ac set at 70 at night so her sinuses don't act up and the hvac vent is on my side. So I am always cold, and she is always bundled up like a mummy. But her sinuses will be a mess in the morning if it is any warmer while she sleeps.