This paper aims to consider the historical development on transitional zone of dome employing triangles in Islamic mausoleums built from the second half of the 12th to the first half of the 15th century in Anatolia. When looking at the transitional zones of the target 50 mausoleums, the transitional zones can be classified into five types from their morphological characteristics (Table 1). Their morphological characteristics, and their regional and chronological distributions were discussed in the section 2. Based on the results of the section 2, the relationship between each transitional zone were considered with paying attention to the location, the foundation date, and the lower and upper end of transitional zone in the section 3 (Fig. 24). “Small Triangle” placing a small stone with two notched triangles were used at each corner of the octagonal lower end of the transitional zone in the earliest two mausoleums (No. 42 and 43) of all which were built in the middle part of Anatolia. This transitional zone continued to be used till the first half of the 14th century (No. 46) as it showed minor changes. Focusing on the other mausoleums of which lower end of the transitional zone is octagonal, although No. 2 in the first half of the 13th century had 16 triangles like No. 42, “Turkish Triangle” was used by enlarging each triangles. Since the more complicated “Turkish Triangle” were applied to No. 8 and 9 having the octagonal lower end, it is presumed that “Turkish Triangle” applied to the octagonal transitional zone had evolved more complicated and precise after the second half of the 13th century.
“Turkish Triangle” also began to be applied to the mausoleums of which lower end of the transitional zone is square in the first half of the 13th century. In the 13th century, “Turkish Triangle” which had large and a small number of triangles (No. 3 and 6) or which looks like “Discontinuous Turkish Triangle” (No. 4 and 7) were used. However, it can be pointed out that “Turkish Triangle” became complicated with using a larger number of triangles than before in the 14th century (No. 12 and 13), and it gradually propagated to the west. Contrary to “complicating” mentioned above, “Small Triangle” used for the mausoleums of which lower end of the transitional zone is octagonal showed a change to the simplified transitional zone which used only one triangle at each upper corner of the body (No. 45 and 49). A similar change can be seen in the “Turkish Triangle” applied to the mausoleums of which lower end of the transitional zone is square. While “Turkish Triangle” had been used since the first half of the 13th century, “Discontinuous Turkish triangle” (No. 31 and 32, etc.) was created in the middle part of Anatolia as a different type of “Turkish Triangle” in order to provide four openings in the transitional zone, and then it was introduced into the west as one formulated transitional zone in the 14th century (No. 36 and 39, etc.). Furthermore, “Turkish Fan” was also created in the middle part of Anatolia as a new transitional zone that simply and smoothly compose each upper corner by 2 or 3 triangles (No. 14). In conclusion, the transitional zone employing some small triangles, which is “Small Triangle”, was created first in the middle part of Anatolia. Based on this transitional zone, the various transitional zones, such as “Turkish Triangle”, “Discontinuous Turkish Triangle”, and “Turkish Fan”, were created by the two conflicting methods like complicating or simplifying and their use areas were expanded to the western part of Anatolia over time.