Joggled grooves along the cylinder body – rillor in Swedish, like the hoops pressed into an oil drum at industrial scale – serve three functions in pressure vessel construction. They strengthen the shell against external collapse and internal stress, they drive the cylinder into a more accurate circular cross-section, and – most importantly for boiler and process vessel service – they let the wall absorb pressure and volume changes as gas or liquid inside the vessel expands and contracts under heat and pressure cycling. Without those rolled grooves, stress concentrates at welds and seams.