The red atoms are potassium, sodium, or calcium. Unit Cells Repeating patterns, whether flowers on wallpaper, or atoms in a crystal, can all be described in terms of Unit Cells. A unit cell is an imaginary box that contains the basic pattern. Repeating the unit cells recreates the whole pattern. Note that the rhombus pattern can be considered either as made up of rhombuses or as a rectangle with an extra point in the center.
Crystallographers prefer the latter, because it makes the rectangular nature of the pattern clearer.
Bricks in a wall have this pattern. The hexagonal pattern can be described by rhombuses oriented in one of three ways. Two are in red and the third is outlined but not colored. The three unit cells lead to identical descriptions of the pattern. We can modify a cube by shaving off the edges, as shown in the top row.