Source designations for objects in 2MASS Catalogs conform to the recommendations outlined in International Astronomical Union's Specifications concerning designations for astronomical radiation sources outside the solar system. Source designations should be given as:
2MASxy Jhhmmss[.]s ± ddmmss
The brackets and period are not explicitly in the name, and are shown
above only to illustrate that the last digit before the declination
sign is tenths of RA seconds. The tenths digit is truncated rather
than rounded, per the IAU recommendation.
The x and y in the acronym correspond to characters that
specify the catalog and data release from which the
object was taken, respectively. The acronyms are as follows:
| Acronym | Catalog/Release |
| 2MASSs | For objects in the 2MASS Sampler Point Source Catalog |
| 2MASXs | For objects in the 2MASS Sampler Extended Source Catalog |
| 2MASSI | For objects in the Incremental Release Point Source Catalogs (this release) |
| 2MASXI | For objects in the Incremental Release Extended Source Catalogs (this release) |
| 2MASSW | For objects in the Working Point Source Databases (pre-release) |
| 2MASXW | For objects in the Working Extended Source Databases (pre-release) |
The coordinate sequence is the sexigesimal J2000-equinox RA and declination, hence the "J" designator.
The acronym designation is not carried explicitly for entries in the Catalogs to minimize duplication. The proper designation for objects in the this and all following Incremental Data Releases is 2MASSI and 2MASXI. The coordinate sequence is tabulated for each source and is included in the source record entry in the Catalogs.
Please note that the "cntr" field listed for each entry in the Sampler Point and Extended Source Catalogs is not a valid or unique 2MASS source identifier! Users should not use it as a means of identifying or referring to 2MASS objects.