The stool is probably the most versatile piece of furniture, it is also used as a side table or step stool when an object is out of reach.
Family tree is an intermediate version between the stool and the step stool, the additional level of course makes it easier to climb on the stool but also allows you to put a drink next to you when sitting.
The passage of 3 to 4 feet is both a constructive question on the stool itself and the relationship between two similar but different objects, a questioning on difference and filiation but also a work on balance and proportions.
It also refers to family, to generations, but also on the link with nature, as if the step had grown on one of the stool's feet like an offshoot, similar but different,
which justifies his name Family tree---