It took me a while to realize what this picture on the cover of the book "Introduction to Analytic Number Theory" (Apostol, 1976) really means.
Special thanks to @LavaboMouille for helping me interpret this picture correctly.
On the horizontal axis we have x = 0, 1, 2, ..., 17. On the vertical axis we have y = 0, 1, 2, ..., 17. A cell exists at (x, y) if and only if gcd(x, y) ≠ 1.
In other words, a cell exists at (x, y) if and only if x and y are not relatively prime to each other.
When x = 1, we get gcd(x, y) = 1 for every integer y, so the column at x = 1 is completely empty. Similarly, the row at y = 1 is also completely empty.