Master clutch cylinder. But both suck to not have. I remember vacationing with my family in Colorado Springs, CO and having both clutch cylinders on the car fail as we were pulling into the alley parking lot for the Nikola Tesla museum there. Sucks trying to get a Saab (or any other uncommon foreign car) fixed in a strange town on short notice.

But then, we were at least in something vaguely resembling civilization.