I am not able to figure out how the graphs shown here are constructed?
For example, what does this graph represent?

(source: wikimedia.org)
“Nodes are distribution of discs“, but I will only have one disc of size a. So, what does node aa represent?
I know the answer would be simple, but I cannot figure it out at the moment.
a,bandcare the three pegs. Soaameans that the two discs are on pega.