Indeed for TeX letters are three dimensional objects. TeX lines up characters at their baseline, what goes below the baseline is their depth.


As Feynman once explained there are always different perspectives to out common understanding -- in this case -- the words three dimensional. For metafont there are multitude of dimensions describing a particular letter. Use the package layouts to see the values and generate the image above.
\documentclass{article} \usepackage{layouts} \begin{document} \printparameterstrue \drawfontframelabel{\Huge\textbf{tangling}} \fbox{\Huge m}\fbox{\Huge g}\fbox{\Huge t} \end{document}