Different nomenclatures are used for tooth numbering. The American Dental Association Universal Numbering System is most common in the United States, where the left maxillary second premolar may be referred to as "tooth #13" or "ADA 13". Palmer notation, also known as the Zsigmondy system, is commonly used in the United Kingdom but is difficult to reproduce on computer systems, so the alphanumeric modification is often preferred when typing ¹. The FDI World Dental Federation notation (ISO 3950 notation) is most common in the rest of the world.