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

Lebesgue rectangles are a simple geometric way to visualize how the Lebesgue integral builds up area by looking at slices of the range rather than slices of the domain. Instead of cutting the x-axis into intervals and stacking vertical rectangles, the Lebesgue approach groups together all points where the function takes values within a certain band. For each horizontal “slice” of height Δy, you look at the set of x-values where the function falls inside that slice, and then measure the total len