From the introduction of TOPS in 1973, all British Rail diesel and electric locomotives and multiple units were allocated to a particular traction maintenance depot or TMD. Drawing from the terminology of steam traction, these depots were generally referred to as "sheds", and indeed most locations were those which had possessed depots for steam locos. Each TMD had a two-character code which was originally displayed underneath the number of each locomotive allocated there. Later this was extended to cover each carriage of diesel multiple units (DMUs), and even some coaching stock; at this time additional depot codes were added covering places with coaching stock allocations, and also works locations.