be sent to toll center BB, then to end office E, and then to the intended telephone. The third thing a Class 4 switching center is connected to is an even bigger switching center, which is called (no big surprise) a Class 3 switching center (also called a primary center). If Mona's call is going to a phone not connected to any of the end offices available from toll centers AA or BB, then Mona's speech moves along to the class 3 switching center. The good news is that the Class 3 switching center works in pretty much the same way as the Class 4 switching center. Basically, from the Class 3 switching center, Mona's call can go to: (1) any Class 3, 4, or 5 switching center that is connected to the Class 3 switching center holding Mona's speech-it will go that route if the intended phone is connected to one of these Class 3, 4, or 5 switching centers; otherwise, (2) the call will be switched to an even bigger switching center, called the Class 2 switching center (and also called the sectional center). Let's say Mona's call heads out to the Class 2 switching center. From here it gets to Carl in one of two ways: (1) if Carl's phone is connected to a Class 2, 3, 4, or 5 switching center that is directly connected to the Class 2 switching center containing Mona's voice, then Mona's voice gets sent to that switching center; otherwise, (2) Mona's call will go to the last, biggest switching center, the Class 1 switching center (or regional center). The Class 1 switching center will then send the call to either a Class 1, 2, 3, 4, or 5 switching center that it's connected to, depending on which center is most directly connected to Carl's phone. And that, my friends, is how Mona's concern about Carl's eating habits gets from Mona in Montreal to Carl in Colorado. It's a 5-level hierarchy of switching centers. There are some 13,000 toll centers, 265 primary centers, 75 sectional centers, and 12 regional centers. 2.2.3 The Signals Sent from Switching Center to Switching Center We now understand that POTS is made up of five stage