This comes up when giving fractional coordinates into a grid. Is (1,1) the upper-left corner of the upper-left grid box, the center, or the lower-right (or the center of the next grid box)? Alternately, is the upper-left corner of the upper-left grid box (-0.5, -0.5), (0.0, 0.0), (0.5, 0.5), or (1.0, 1.0). I've seen all four conventions used in different places, depending on whether the grid boxes themselves are numbered starting at 0 or 1, and when extended to fractional if it makes more sense to have the corner or the center of the grid box take the value of the box itself.
