Generally periscopes are used to see the objects which are at a greater height by hiding in ground. The beam of light which is incident on the first mirror must be incident on second mirror after reflection from the first mirror. The mirrors are arranged at an angle of 45°. It is possible only when it is in the shape of 'Z' and in other shapes it is impossible to see the object by hiding. So periscopes are in 'Z' shape.
Eg: Let us assume that periscope is in "T" shape. It is impossible to see the object by hiding.