All elements of a computer image consist of pixels (picture elements), which are to say small areas, dots, spots, or however one wishes to describe them. A pixel is the smallest addressable unit in an image. That is, it can be uniquely located using a horizontal coordinate value X or i and a vertical coordinate value Y or j. We usually specify these values as integers, precisely because they are the smallest addressable unit, and so there can be no pixel between two adjacent pixels. Note that computer graphics typically use a coordinate system where (0,0) is at the top-left corner, with Y values increasing downward, which differs from standard mathematical coordinate systems.