The B650 was a short link road in north Middlesex. As the lowest-numbered Class II road in the 6 zone its position was right at the southern end of the 6 zone.
It started on the A6 in South Mimms and headed eastwards into Potters Bar where it ended. Originally it ended on the A1 but that road was diverted to the west after World War II and its old route became the A1000; the B650 therefore crossed the A1 fairly close to its western end.
By the 1970s the B650 had become an extension of the A111, which it also met in Potters Bar. However, the construction of the M25 renumbered many of the roads in this area - as the A111 no longer connected to a Class I road it was downgraded and became an eastern extension of the B556, which it remains.