Ekagami-shiki Kofun (hand-drawn mirror-style burial mound)

… The characteristics of the form of keyhole-shaped tumuli changed over time, and they played an important role in the chronological study of ancient tombs. Many of the early ones were located on the edges of hills or plateaus, and were characterized by a low, narrow front section attached to the front of a high circular rear section, and some are called "handle mirror-style tumuli." However, it has been pointed out that in the oldest group, there are many examples in which the front section opens wide in a plectrum shape from a narrow, constricted section. …

*Some of the terminology used to refer to "Hakagami-shiki Kofun" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…　前方後円墳の形態は，時期によってその特徴が変化し，古墳の編年的研究に重要な役割を果たした。前期は丘陵上や台地の縁辺部に立地するものが多く，高い後円部の前面に低くて細い前方部が付くのが特徴で，一部は〈柄鏡式古墳〉と呼ばれる。ただし最古の一群には，前方部が狭いくびれ部から撥（ばち）形に大きく開く例の多いことが指摘されている。…

※「柄鏡式古墳」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報