The primary beam limits the size of an object that we can observe with a conventional experiment. To circumvent this, a large object can be observed using multiple pointings - this is the practise known as mosaicing. In interferometry, mosaicing is not simply the practise of pasting together multiple tiles of the sky. In interferometry, the adjacent pointings are not independent, and so we can get fundamentally better images by processing the different pointings together. This is particularly so for extended emission or when the signal-to-noise ratio is low. A description of the theory behind mosaicing can be found in the NRAO Synthesis Imaging Summer School (Lecture 15 - Wide Field Imaging III: Mosaicing - by Tim Cornwell). Other notable references are Cornwell (1988) (A&A, 143, 77) and Cornwell, Holdaway & Uson (1993) (A&A, 271, 697). Sault, Staveley-Smith and Brouw (1996) (A&AS, 120, 376) describe some of the algorithms used by Miriad.