Fourier modes of a 3D cube and a 2D map (upper and lower subplot respectively), in the manner of Ponthieu et al. (2011). The power spectrum P(k) at a given k value is the average of all spatial frequencies with the same magnitude, that is concentric shells or rings around the centre where k = 0. The inner shell or circle in red, which touches the edges of the box, is the largest k value where all k modes can be included. All k values with higher magnitude, such as the outer shell or circle in green, lie outside the box and thus miss k modes, introducing greater uncertainties in the corresponding power spectrum value. In these examples k is unit-less.