With an approximate diameter of 345,000 light-years, NGC 6331 is well considered one of the largest galaxies.[3] It is the brightest group member of Abell 2256 and first in 6' string to the east, making up of 6 galaxies.[6] NGC 6331 also makes up a part of the galactic triplet CGCG 355-024[7] which comprises two other elliptical galaxies in the cluster, CGCG 355-024 NED03 (PGC 59513)[8] and CGCG 355-024 NED01 (PGC 84827).[9]