The D-Class

Mercedes-Benz offers the A-Class, B-Class, C-Class, E-Class, etc. … but no D-Class. Is it because the size of a sedan between the C-Class and the E-Class wouldn’t make sense? Or could it be that “D” carries with it bad connotations in Germany? Or are these letters just an arbitrary set of labels, with no meaning behind them? Is this even worth blogging about?