We then look for other transformations that leave the pattern unchanged.
We find three types:
vertical reflections through the centre of each brick, horizontal reflections through the centre of
each brick, and 180° rotations.
At the beginning diagrams (e.g., Fig. 2) are sufficient to describe
the locations of mirror lines and centres of rotation .
I am always modifying the activities I use,
and so when I introduce precise language depends on the students and on the way the activities
have developed.
When the time comes I distinguish between elements of the bricks, and elements of the pattern. Most importantly, the four segments bordering a brick are its sides, but the sides of
a brick might contain several edges of the pattern, an edge being a segment joining two adjacent
points (this terminology is used by Grünbaum and Shepard [1987] to describe tilings of the
plane).
For example, in Running bond each brick is bordered by six edges, as the upper and lower
sides of each brick are made up of two edges. The rotations occur at the midpoints of edges and at
the centre of each brick (see fig. 2).
Those at the midpoints of vertical edges and at the centre of
each brick are compositions of the two reflections.