A Tom Thumb typically refers to a short shanked curb or pelham bit with swiveling cheeks. It is a mild type of curb, since the shorter cheeks limit the amount of leverage that can be used. Most curbs today come with the option of getting them with longer or shorter shanks. To get an idea of what constitutes a shorter shank, an average length of lever (below mouthpiece) is around 7cm (2 3/4 inches), while short would be around 5cm (2 inches).