This article (17 pages) was written to accompany an Overleaf help item. It explores the meaning of underfull or overfull boxes and examines the calculation of badness using an \hbox with finite and infinite glues. We have tried to provide a range of examples to demonstrate various aspects of TeX engines’ box-construction behaviour and the underfull or overfull “diagnostic messages” issued by TeX engines during the box-creation process.