Have you ever been lost in the city for an “M”? Does every city use “M” to represent where the metro entry is? The answer would be that not all cities call it metro. In the UK there are S for Glasgow subway, and the London underground or the tube. Similarly in German-speaking countries is known as U-Bahn (short for Untergrundbahn) and thus U is used in signs. Finally in the north, the letter you should look for is T: in Stokholm is Tunnelbana and in Oslo Tunnelbane.
The most common letter in the world is “M”. However some cities instead of a single letter, they use a full or a partial world. For example, the word “Metro” in Madrid. In Paris, you can find this word along with “Métropolitain“, “Metra” and, “M”. Also in London, “Underground”. Some signs are not in Latin script. The metro in Yerevan (Armenia) looks like a U or a V, but it is actually an “M” in the Armenian script.
Other transit systems do not use any letter but a symbol. In Europe, it is the case of Bilbao’s metro.