In Lebesgue integration, a Lebesgue rectangle is one of the basic building blocks used to measure sets or approximate integrals. A rectangle in this sense does not always mean a shape you can draw on paper, but rather a product set formed from two intervals on the real line. For example, you can think of it as taking one interval along the x-axis and another along the y-axis, and then combining them into a rectangle-shaped region. These rectangles are used to cover parts of the domain of a funct