memes

Yes, the C/blue wire is common, basically a neutral for the 24v system and necessary for more digital thermostats to keep the thermostat powered (some can work without the c wire, but it depends on the unit feeding power). The old mechanical ones work on the tilting mercury thing or copper coil for temp sensing, and only require the red wire to touch their respective wires to call whatever function, but the digital ones do the same thing on a switching level. I know there are additional wire sometimes for multistage heat and zoning, but as far as I know it's the same principle. I'll be honest I'm an electrician by trade and not an HVAC guy, and I know some of the more intricate systems can deviate from this, but your average residential system should be similar or damn near the same as my original comment (granted my experience is in Southern CA, so there are possibly regional differences with oil furnaces, radiator systems, etc).