In this paper we review the literature on visual constraints in written word processing.We notice that not all letters are equally visible to the reader. The letter that is mostvisible is the letter that is fixated. The visibility of the other letters depends on thedistance between the letters and the fixation location, whether the letters are outer orinner letters of the word, and whether the letters lie to the left or to the right of thefixation location. Because of these three factors, word recognition depends on theviewing position. In languages read from left to right, the optimal viewing position issituated between the beginning and the middle of the word. This optimal viewingposition is the result of an interplay of four variables: the distance between theviewing position and the farthest letter, the fact that the word beginning is usuallymore informative than the word end, the fact that during reading words have beenrecognised a lot of times after fixation on this letter position, and the fact that stimuliin the right visual field have direct access to the left cerebral hemisphere. Forlanguages read from right to left, the first three variables pull the optimal viewingposition towards the right side of the word (which is the word beginning), but thefourth variable counteracts these forces to some extent. Therefore, the asymmetry ofthe OVP curve is less clear in Hebrew and Arabic than in French and Dutch.