Bagua is a 8-angular symbol, that corresponds to the four main-directions and the four inter-directions. The 8 Trigrammes come from the I-Ging. The Yin Bagua is that of the early heaven and the Yang Bagua that of the late heaven.           
   
The Yin-Bagua is used for the Bagua mirror, who turns away incoming bad energies.    
   
Convex means curved outside and scatters the incoming Sha-Chi   
Concave means curved inside and collects the incoming  Sha-Chi   
   
The flat mirror sends back the incoming Sha-Chi to his cause. 

Diameter 13 cm