 These differed from modern saunas in that they had no windows and were heated by heating up a pile of rocks (called "kiuas") by burning large amounts of wood for about 6–8 hours, and then letting the smoke out through a hatch before entering to enjoy the sauna heat (called "löyly").The tradition of wood construction - beyond the "kota" hut - has been common throughout the entire northern boreal coniferous zone since prehistoric times.The central structural factor in its success was the corner joining - or "corner-timbering" - technique, whereby logs are laid horizontally in succession and notched at the ends to form tightly secure joints.