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