The MERGE storage engine, also known as the MRG_MyISAM engine, is a
collection of identical MyISAM tables that can be used as one.
"Identical" means that all tables have identical column and index
information. You cannot merge MyISAM tables in which the columns are
listed in a different order, do not have exactly the same columns, or
have the indexes in different order. However, any or all of the MyISAM
tables can be compressed with myisampack. See
http://dev.mysql.com/doc/refman/5.5/en/myisampack.html. Differences in
table options such as AVG_ROW_LENGTH, MAX_ROWS, or PACK_KEYS do not
matter.

URL: http://dev.mysql.com/doc/refman/5.5/en/merge-storage-engine.html

