By volume of content it's "usually" called the rule of 3, because the rule of 5 didn't exist until C++11, and most of the C++ content was written before moving

By volume of content it's "usually" called the rule of 3, because the rule of 5 didn't exist until C++11, and most of the C++ content was written before moving was a thing. It's now called the rule of 5.