An infinitely long sinusoid has a Fourier transform that is non-zero at only one frequency. Multiplying that sinusoid by any window function is a non-linear operation that always creates new Fourier transform components (leakage), whether or not the number of cycles in the window length is an integer. But when the window is rectangular and the number of cycles in the window length is an integer, it is possible to sample the transform (a sinc function) at only the main lobe and the zero-crossings, leaving the false impression of no leakage. But the unsampled sidelobes are frequencies that undesired signals can leak into and distort the desired one.

An infinitely long sinusoid has a Fourier transform that is non-zero at only one frequency. Multiplying that sinusoid by any window function is a non-linear operation that always creates new Fourier transform components (leakage), whether or not the number of cycles in the window length is an integer. But when the window is rectangular and the number of cycles in the window length is an integer, it is possible to sample the transform (a sinc function) at only the main lobe and the zero-crossings, leaving the false impression of no leakage. But the unsampled sidelobes are frequencies of potential other signals that will be distorted by the sinusoid and (conversely) frequencies of other signals that can leak into and distort the desired one.

An infinitely long sinusoid has a Fourier transform that is non-zero at only one frequency. Multiplying that sinusoid by any window function is a non-linear operation that always creates new Fourier transform components. That is "leakage", whether or not the number of cycles in the window length is an integer. If that Fourier transform is a sinc function (rectangular window) and we sample it at only the main lobe and the zero-crossings, then we will see the illusion of no leakage. But we will be quickly disillusioned when we apply that rectangular window to our sinusoid embedded in a summation of other signals.

An infinitely long sinusoid has a Fourier transform that is non-zero at only one frequency. Multiplying that sinusoid by any window function is a non-linear operation that always creates new Fourier transform components (leakage), whether or not the number of cycles in the window length is an integer. But when the window is rectangular and the number of cycles in the window length is an integer, it is possible to sample the transform (a sinc function) at only the main lobe and the zero-crossings, leaving the false impression of no leakage. But the unsampled sidelobes are frequencies that undesired signals can leak into and distort the desired one.