Shapes that are not completely closed can also be perceived as a gestalt. A closed shape is always seen as being on the inside of the linking line - the formative effect therefore only works in one direction. This inner side is usually identical to the concave, surrounding side of the line that encloses the shape. This in turn leads to a formative effect even in the case of arcs or angles, making a pure shape visible inside the line, that is to say, in the partly enclosed area. If this leads to a plausible interpretation of the
initial pattern, the effect of the inner side can be significant.

An arc makes a pure shape visible on the inside of the line.