The Regelbau (German for "standard design") were a series of standardised bunker designs built in large numbers by the Germans in the Siegfried Line () and the Atlantic Wall as part of their defensive fortifications prior to and during the Second World War. Purpose There were several advantages to standardising the design and construction: Retention of proven design features when building new bunkers Simpler manufacture of large quantities e.g. of armoured and ventilation components and simplified ordering from the industrial manufacturers involved Easier surveying of construction sites with regard to the tactical situation Simplified construction process Easier supply of materials to the construction site Extent of standardization As early as 1933 the Army command began work on the standardisation of defensive works with their publication of the Order for the Construction of Permanent Fortifications (Vorschrift zum Bau ständiger Befestigungsanlagen) or B. st. B. In addition to general directions, it also contained very specific regulations on the armoured components (Panzerungsteilen or P-Teile) and ventilation components (Lüftungsteilen or ML-Teile) to be used. The most important aspect of standardization was construction thickness (Ausbaustärke). This referred to the thickness of the walls and ceilings of the bunker. In the four years it took to build the Siegfried Line, changes were repeatedly made to the list of components to be used. The reasons for this were continual developments in weapon technology as well as the availability of armoured components and raw materials (steel) in general. Construction thicknesses (wall and ceiling thicknesses): A = 3.5 metres A1 = 2.5 metres B new = 2.0 metres B old = 1.5 metres B 1 = 1.0 metres C = 0.6 metres D = 0.3 metres In addition, bunker design was arranged in "series", the 1939 designs with A and B thickness falling into the 100 series. Later 400, 500 and 600 series were created, the new series were more designed to enable the casemate to be capable of taking captured weapons than to be stronger, the 400 series was designed for Czechoslovakian weapons. The Channel Islands received mainly 600 series constructions. Whilst constructions were undertaken and planned by the OT for the Heer (army), the Kriegsmarine (navy) and Luftwaffe (airforce) would also man some of the fortifications, and they had their own designs and designations of fortifications, the Navy used M (Mittlere or Medium) for normal sea defence batteries with S (Schwere or Heavy) for the larger calibre