I've always loved the way that in many metro stations in Paris, the tiling scheme includes bespoke frames for posters. It was great, then, to happen upon an information board explaining their history: advertising was a planned feature of the metro from its very beginning. When the Nord-Sud company opened its network in 1910, it included these ceramic frames in its stations. Its competitor the CMP followed its example in 1922.
I couldn't resist the advertisement for Breton strawberries in the picture above. Some details from various frames are illustrated below.