Back to back houses were simple workers houses that, to save space and cost, were built in Victorian times and earlier. They shared a back wall hence being called 'back to back'. They often only had one room downstairs and one upstairs, with no toilet or running water. They were built in the poorest areas and suffered from over crowding and unsanitary conditions.
A few examples still exist today in Birmingham and Leeds but most have been cleared in slum clearance program's over the last 50 years since the Second World War.
The odd example has been preserved as part of living Museums of social history.