. . . accomplished in building several forms of log housing, having different methods of corner timbering, and they utilized both round and hewn logs. Their log building had undergone an evolutionary process from the crude "pirtti". . . a small gabled-roof cabin of round logs with an opening in the roof to vent smoke, to more sophisticated squared logs with interlocking double-notch joints, the timber extending beyond the corners. Log saunas or bathhouses of this type are still found in rural Finland. By stacking tree trunks one on top of another and overlapping the logs at the corners, people made the "log cabin". They developed interlocking corners by notching the logs at the ends, resulting in strong structures that were easier to make weather-tight by inserting moss or other soft material into the joints. As the original coniferous forest extended over the coldest parts of the world, there was a prime need to keep these cabins warm. The insulating properties of the solid wood were a great advantage over a timber frame construction covered with animal skins, felt, boards or shingles. Over the decades, increasingly complex joints were developed to ensure more weather tight joints between the logs, but the profiles were still largely based on the round log.