Sekizoku is classified into three structural types depending on the shape of ki-hen (the base); Hei-ki (flat base), which had a straight base, concave base, which had a depression, and convex base, which had a projection.
石鏃を、底辺（基辺）の形状が直線になっているものを平基、くぼむものを凹基、突出するものを凸基の3種に分けられる。