However, a closer look at the slanted sides of the trapezoidal and triangular pieces shows that they cannot be aligned as implied in the above fallacious illustrations. In fact, they are the diagonals of two dissimilar rectangles of sizes 2x5 and 3x8, respectively, and hence have distinct slopes. But the difference of the ratios (2/5==0.4 versus 3/8==0.375) is too small to be perceived by the eye.
Note that the dissection cuts the sides of the 8x8 squares according to the proportion 5:3. The illusion becomes even more effective if the numbers 3, 5, 8 are replaced by a triple of higher consecutive Fibonacci numbers.
Dissection Fallacy -- from Wolfram MathWorld