A basic rule that is usually but not always correct.
As a rule of thumb, plant tomato seeds three inches deep.
Based on the use of ones thumb as a rough measurement tool. Generally correct for course measures.
Most old English measures of distance were based on the body measurements of the king - the length of the foot, inch (thumb tip to first knuckle), cubit (elbow-to-fingertip), and yard (nose-to-fingertip).
Alternative: Old English law (and potentially early American law) declared that it was okay for a man to beat his wife but had to use a stick no larger in diameter than his thumb.
There is no truth to the rumor that this law was an early start to the women's rights movement.