As for the reason for the triangular-rim, there are some theories that most of mirrors were convex and it was structurally easier to make them with a triangular-rim, or that wooden fences surrounding a holy place were the model.
なぜ、三角縁とするかの理由については、ほとんどが凸面鏡であり、三角縁にすると構造上作りやすいから、あるいは、神聖な場所を囲む瑞垣をまねた等の説がある。