This paper deals with large scale aspects of Hill's equationẍ +(a +bp(t))x = 0, where p is periodic with a fixed period. In particular, the interest is the asymptotic radial density of the stability domain in the (a, b)-plane. It turns out that this density changes discontinuously in a certain direction and exhibits and interesting asymptotic fine structure. Most of the paper deals with the case where p is a Morse function with one maximum and one minimum, but also the discontinuous case of square Hill's equation is studied, where the density behaves differently.Mathematics Subject Classification: 34C08, 34D23, 26B15, 54C30