Tektronix 536 Oscilloscope
Of all the big box Tektronix oscilloscopes, the 547 like the 536 was perhaps the most popular. It was a single beam oscilloscope costing $1875 in 1968. Even with four dual-beam scopes in the lineup, the 547 was made popular largely because of the innovative "ALT" mode, which allowed for dual traces to be shown on a single beam oscilloscope, providing much of the functionality of dual-beam scopes for a fraction of the added cost.
It was of interest to many users to be able to see more than one electrical signal on the screen at the same time, so they could be easily compared. There were two methods of doing this. One was a dual-beam oscilloscope. In a variety of ways, one oscilloscope was really two, the two scopes coming together inside the crt, where two electron beams were directed independently onto a single phosphor screen. You can imagine that this would be a very expensive approach, as two of everything was required. Tektronix oscilloscopes like this were the 502, 551, 555, and 556. Some of these had two time bases and two sets of horizontal deflection plates, so the horizontal scan of the two beams could be synchronized (triggered) separately from the other.
Another way to do the same thing was to use an electronic switch in front of the vertical system in a single beam oscilloscope. This is called a "dual trace" oscilloscope. The switch worked to display 2 or 4 separate electrical signals on the screen. This required little duplication and added only a small amount to the cost. This is the technique used in all the dual and four-trace plug-ins such as the CA, 1A1, 1A2, Type M and 1A4.