Limitations in powder diffractometry imposed by scatter slits and/or a diffracted-beam monochromator, which have been ignored in the past, are discussed and mapped quantitatively. These limitations become manifest especially with ωoffsets, i.e.in stress measurements and with ωoscillation to improve the reproducibility of intensities in the presence of too coarse grains. The limitations can only be established with knowledge of slit sizes and with precise alignment of all slits, their holders and the diffracted-beam monochromator. To that end, concise, accurate procedures for obtaining these measurements in-situare proposed.