Apostrophes are used to create a common branch between two well-established branch points in the tree. One example is M1’20’51. It is the common parent of the M1, M20, and M51 branches.
Note: Some branches of the tree that were joined before the apostrophe was adopted are joined without the use of an apostrophe by historic convention. Examples are HV, which was, when named, the parent of the H and V branches and JT that is the parent of the J and the T branches.