The majority of tunes are written in two-part form with both sections repeated once, denoted commonly by the letters AABB. It is quite common, however, for tunes to have additional parts, for one or more parts to be played without repetition, or for other orders of parts to prevail, thus AAB, AABC, AABBCCDD, ABCB or other combinations may readily be found. Frequently, various printings or recordings of a given tune will have different patterns repetition of parts or will have added or deleted parts compared with other versions of the same melody, depending on a number of factors including regional variations and the whims of individual musicians. Thus the patterns of parts in the index are represented and identified by source, as AABB (O'Neill): AABB (Breathnach), unless the tune is consistently played with one particular pattern. Repetitions of tunes which end in different cadences, have different leading or transitional notes, or feature other variations are recorded by the convention of an apostrophe: AA'BB'