Crystallite size is the mean extent of coherently diffracting domains within a powder or polycrystal, a quantity distinct from particle size because one particle may contain many such domains. Diffraction line broadening is the usual access route: the Scherrer relation converts peak width into a domain dimension, and its extensions recover a size distribution rather than a single number [1]. Size broadening overlaps with strain broadening in the same profile, so separating the two requires several orders of reflection or a full-profile treatment [2]. Different routes to the same quantity give systematically different answers, as comparisons on hydroxyapatite show [3], and the same caution applies when the phase sits in a deformed matrix [4]. Whether the value should be read as a physical domain dimension or as an instrumental index of peak width depends on how much of the profile has been modelled; current surveys place these classical treatments alongside machine-learning analysis of diffraction data [5].