Amenities: as dwellings became more and more comfortable, this was echoed in the plaques on the façades; these plaques were a form of advertising since most buildings were for rental: "water and gaz on all floors" is obviously better than "water in the house" and "connection with the sewers" ensured better hygiene and fewer smells; "lifts" (which means having electricity) made the upper and therefore lighter floors more desirable. These plaques were also symbols for the occupiers who wanted to stand out above the common herd.
Poking fun: some witty Parisians have wanted to remind us that glory is not only for "celebs".
Spelling variations: the plates may reserve many surprise in the use of the spelling.
Stick no bills: or the many ways of enforcing the law of 1881.
Street plaques: blue plaques with white lettering indicating the names of Parisian streets are not always blue and white letters not always white! when they are not missing, the districts are in Roman or Arabic numerals; and there are many other variations: the shape and framing, the fonts, uppercase and lowercase ...
Miscellaneous: walls speak, even they are sometimes enigmatic.