Small circles mean rotations of order 2. Mirrors are represented by red lines, glide reflections by green lines. Yellow lines are the border of a fundamental region. (12) means that the colours white and grey are interchanged; i means the identity.
This is an example of the case stated in the fourth row of the table in Section 4.5 (Figure 13, «p2mg») in
Wallpaper groups. Portuguese pavements: Adro da Igreja do Senhor Bom Jesus de Matosinhos (Igreja Matriz) - p2gg
O Nasoni era italianíssimo, mas soube entender os méritos do granito lusitano...