The brand name "S-Bahn" chosen in 1930 mirrored "U-Bahn", which had become the official brand name for the Berlin city-owned rapid transit lines begun under the name of "Berliner Hoch- und Untergrundbahnen" (Berlin elevated and underground lines), where the word of mouth had abbreviated "Untergrundbahn" to "U-Bahn", in parallel to "U-Boot" formed from "Unterseeboot" ("undersea boat" – submarine).