This is a 8x8 chessboard
We can clearly see that it consists of 8 Rows & 8 Columns. And 8 Rows & 8 Columns result in 9 Horizontal Lines & 9 Vertical Lines.
To form a rectangle we have to select 2 Horizontal lines out of 9 Horizontal lines & 2 Vertical lines out of 9 vertical lines.
So, total number of rectangles Iin this 8x8 chessboard = 9C2 * 9C2