The practice of rigidly defending front-line trenches, regardless of casualties was abolished, in favour of a mobile defence of the fortified areas being built over the autumn and winter of 1916–1917. (Principles of Field Fortification) was published in January 1917, in which instructions were given for the construction of defences in depth, according to the principles of greater depth and of disguise by dispersal and camouflage. Trench-lines were mainly intended for accommodation, dumps of supplies and as decoys, rather than firing lines. Deep dug-outs in the front line were to be replaced by many more smaller, shallow (MEBU shelters) with most built towards the rear of the defensive areas. Within the new forward zones, battlezones and rearward battle zones, the chain of command was streamlined by making corps headquarters into (groups), responsible for the administrative tasks in an area into which divisions would be moved for periods, before being withdrawn to rest, train and be brought up to strength. Command of areas rather than units was also introduced in divisions, with command of regiments devolved to the front battalion commander (KTK ), which reduced the chain of command from five to two posts.