The Level 1 Specifications stipulate a sky coverage >95% with no gaps greater than 200 sq. deg.

The 2MASS scanning strategy was devised around 59,650 "tiles" with dimensions of roughly 6° x 8.5´. Observations of these tiles ensure coverage of the entire sky. Tiles were laid out such that adjacent ones would provide overlap of 10% (approximately 50´´) in Right Ascension and 8.5´ in Declination. Three effects are responsible for decreasing the sky coverage from 100%:

• Holes in sky coverage: Although scans of all tiles were obtained during the course of the Survey, not all scans fell perfectly on their intended tile positions. As a result, on rare occasions slight gaps would arise between adjacent scans, either along the east-west edges in RA or along the north-south edges in Dec. Although the presence of such gaps was monitored throughout the course of the Survey, certain "gap fill" observations were never successfully obtained at a quality high enough for inclusion in the catalogs or image atlases.
• Effective gaps between scans: The criteria for Catalog source selection stipulate that a point source be further away than 10´´ from a scan edge and an extended source be further away than 15´´. For scan pairs that overlap by less than 20´´, there will be an effective sky coverage hole in the PSC. Similarly, for scan pairs that overlap by less than 30´´, there will be a hole in the XSC.
• Bright stars: It is not possible to probe to the Level-1 sensitivity specifications in the haloes and diffraction spikes around these objects. This area is not lost entirely to the PSC however; objects bright enough to be distinguished from bright star debris can still be astrometered and photometered, though with reduced accuracy. Completeness in such regions is, nevertheless, reduced relative to parts of the sky lacking bright stars. Detailed sky coverage maps for the XSC are available here.