Due to advancements in centering techniques and the use of brick ribbing, Roman domes proliferated in the 4th century. During the fourth and fifth centuries, the preferred building material progressively shifted from stone or concrete to lighter brick in thin shells. This allowed for the construction of larger buildings using the same number of bricks. The dome itself became flatter and thinner until, by the sixth century, it was possible to see straight sides on some buildings. This may have been due to increased expertise or perhaps the use of darker bricks which would have made the structure appear straighter. From the sixth century onward, domes began to be built with ribs instead of pillars. This reduced the amount of material needed for the dome's support and also allowed more space inside the building below. By the ninth century, half-domes appeared, consisting only of the base section with its flat top half. This simplified design meant that less brick was required and could be constructed faster than a full-scale dome. Half-domes were popular in Muslim countries where they were used as worship rooms called muqarnas. These are still seen today in Iran and Turkey. In Europe, masonry domes were replaced during the 11th century by wooden ones. This may have been done to save money or because wood is a more flexible material than brick or stone.