How does the 4017B Decade Counter work?
The Johnson counter (4017) is a form of ring counter that takes the NOT Q output of the last stage and loops it back to the D input of the first stage. It shifts the data rather than counts it. A binary counter would only require 4 stages up to a 16 count. A 4 stage Johnson counter would only count up to 8.

 

One advantage the Johnson counter has over the binary counter is how it decodes each output. The 4 stage binary counter requires inputs for all four outputs. The Johnson counter only needs 2 inputs to a simple AND gate to produce a unique output.