"...Just like the Tamils of South India, the Tchokwe people invented a similar mnemonic device to facilitate the memorization of their standardized drawings. After cleaning and smoothing the ground, they first set out with their fingertips an orthogonal net of equidistant points. The number of rows and columns depends on the motif to be represented. By applying their method, the Tchokwe drawing experts reduce the memorization of a whole design to that of mostly two numbers and a geometric algorithm. Most of they drawings display bilateral and/or rotational (90^o or 180^o) symmetries. The symmetry of their pictograms facilitates the execution of a drawing. This is important as the drawings have to be executed smoothly and continuously. Any hesitation or stopping on the part of the drawer is interpreted by the audience as an imperfection and lack of knowledge, and assented with an ironic smile." 

P.P.J.Gerdes: On ethnomathematical research and symmetry, 
Symmetry: Culture and Science 1, 2 (1990), 154-170.