These are the walls that are shared by neighbours. In a terrace Neighbours often own half the wall, but they are reliant on the full wall to act as one. Some walls are owner by one side but support the Neighbour’s roof and floors.
The party Wall Act provides a mechanism with which a procedure is set out that allows works to boundary lines and shared walls. A party wall is typically a wall which both you and your neighbour reply upon. A wall that should your neighbour removed re affect the stability of your property.
Two Shakespearean lovers who chatted through a wall lend their name to the Learned Party Wall Society. Croft staff are members of the society and attend meetings and lectures on Party Wall matters; surprising their Christmas lecture is always a real Hoot!