Switches in series are what control circuits are all about.
Many different controls are switches. Think for a moment and see how many you can come up with. (For a review of switches, you might want to look online at www.PMmag.com for my March 2003 column, "Switches Is Just Switches.")
Here's what comes to my mind. There's my favorite switch - the thermostat. There's the switch portion of the relay (more on relays in an upcoming column). And there are all of those switches called limits - high limit switches, low limit switches, aquastats, low water cut-off switches, flame rollout switches and more.