"Ubuntu is based on a snapshot of the "unstable" tree of Debian, which is merged into Ubuntu's current code release every six months. Ubuntu is Debian unstable

"Ubuntu is based on a snapshot of the "unstable" tree of Debian, which is merged into Ubuntu's current code release every six months. Ubuntu is Debian unstable with some of the rough edges smoothed over" I'd say that Ubuntu is more of a fork of debian/testing than debian/sid. The only serious Sid distro I know of is Sidux.