The most obvious change in the Developmental Pueblo Period, as compared to the preceding Basketmaker, was a gradual shift in the type of house construction. The single-unit mud, slab, and jacal semisubterranean house was giving way to the huge multistoried stone and adobe structures, which were to predominate in the Great Pueblo Period 250 years later. In some areas, even earlier than A.D. 750 a few people began to build single-room houses aboveground in a contiguous arrangement, often crescent-shaped, forming small villages. The construction varied from district to district. Some houses were quadrangular in form and wholly aboveground, made of adobe and mud with upright, supporting posts; others were still semisubterranean; some even showed the beginnings of true stone masonry. At this time also, a new type of structure was coming into existence (though a few examples are known from late Basketmaker times). This new structure, the kiva, was simply a modification of, and change in, the use of the old pithouse. A kiva is a ceremonial room and clubhouse for the men, usually constructed underground (or, where aboveground, so clustered in other rooms as to appear belowground in its relation to the surrounding rooms). It is circular like the early pithouses, but normally contains a fireplace, a deflector (to prevent the draft from fanning the fire too much), and a ventilator shaft by which to bring in the fresh air. A “sipapu” (a small hole which supposedly leads to the underworld) was located in the floor on the opposite side of the fireplace from the deflector. Usually there was a bench around the inside of the kiva near the floor, which may either have been used as a place on which to store religious objects and other paraphernalia or may have served the functional purpose of strengthening the lower part of the kiva wall. Smaller kivas frequently had pilasters built upon the bench and extending upward a short distance; these supported the cribbed roof structure. Large kivas had four centrally located posts which helped support the roof. Entrance to a kiva was normally gained by means of a ladder through the central smoke hole in the roof. [Illustration: GROUND PLAN OF A TYPICAL KIVA] [Illustration: CROSS SECTION OF A TYPICAL KIVA 1. SIPAPU 2. FIREPIT 3. DEFLECTOR 4. AIR SHAFT (VENTILATOR) 5. PILASTER (ROOF SUPPORT)] It is difficult to assign the same dates to this Developmental Pueblo Period in all areas of the San Juan Basin. Culturally some sections seemed to lag behind others; some ideas, concepts, and artifacts spread and were accepted faster than others. Also, certain regions have been much better explored archeologically, and we know more about them.