 Exact formulas are known for enumerating polyominoes of special classes, such as the class of "convex" polyominoes and the class of "directed" polyominoes.The definition of a "convex" polyomino is different from the usual definition of convexity, but is similar to the definition used for the orthogonal convex hull.A polyomino is said to be "vertically" or "column convex" if its intersection with any vertical line is convex (in other words, each column has no holes).