3 The centered square structure

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This is a strange choice of indexing for this crystal. What would be a better choice?

You can measure distances and directions in the crystal structure as follows. Place the cursor at the center of one atom, press the left mouse button, move the cursor to the nearest atom down and to the right, and release the mouse button. The display above the left hand frame gives the length and orientation of the line you just drew. (Click once anywhere in the crystal window to erase the line.)