If you mean this type of keys
http://www.keyline.it/images/normProd/chiavi-patent-e-italian-style/chiavi-patent-e-italian-style_57427.jpg
which are used for doors inside the home , then they have numbers from 1 to 9 maybe , its the case in my apartment and if you have a key numbered 3 for a door with same number then it will open