This is a proportional mensuration that is in substance Tempus perfectum Prolatione Imperfecta with the tactus shifted from the semibreve to the breve. The most common time signature present in the Odhecaton, it is a later development than the basic four, and is called `alla breve' because of the shift in focus from semibreve to breve. The mensuration sign for alla breve is a broken circle with a vertical (or sometimes angled) line through it, . Also known as proportio dupla, it can be thought of as a division of time values by two, so that the semibreve in Tempus Imperfecta Proportione Imperfecta (marked with the broken circle which looks ) is equal in duration to the breve in Proportio Dupla. This symbol and term have survived into the present day.