Wales has many old houses built of timber in areas where there was not much stone. The gaps between the timbers were filled in with plaster or brick. The dark timber and white plaster had a strong visual effect. This made the builders think about the patterns made by the timbers and add decorative motifs.
This house at Berriew in Powys was built by the vicar Thomas Kyffin in 1616 – he put the date and his initials over the porch. You can see the vertical and horizontal timbers of the box frame that held up the house, with equal-size oblong panels in between. The sloped timbers at the corners added strength. Above the porch there were squares instead of oblongs. Every square had a shape made up of four curves.
Later, people imitated the tradition of black and white decoration even when houses were not made of timber. A famous example is Plas Newydd, Llangollen. The house was built of stone but the owners stuck timber on the front. You can analyse the rules for the pattern. How many horizontal rows are there? How many verticals are in each row? What motifs are used and where?
© Manfred Heyde - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2825903