A subdomain Ω contained in a
domain A in the Riemann sphere is called a relative circle domain in A if each
component of A − Ω is either a closed disk or a point. Let Ω be a relative circle
domain in the unit disk U in the complex plane ℂ; and let A be a simply connected
proper subdomain of ℂ. Then Ω is conformally homeomorphic to a relative circle
domain in A.