Supernova are designated by their year of discovery and a letter signifying their order of discovery during the year. Example, the supernova in the Large Magellanic Cloud is called '1987A' because it was the first supernova detected in 1987. The next supernova discovered anywhere in the sky that year was 1987B followed by 1987C, 1987D etc. The discovery of a supernova in the Andromeda Galaxy would be called '1996C' if it was the third supernova discovered in 1996, or '2015J' if it was the 10th supernova discovered in the year 2015.