The present disclosure relates to a first connecting element for use in combination with a second connecting element, the first connecting element and second connecting element can be manually connected to and disconnected from each other.  More specifically, the present disclosure relates to such a first connecting element as can be used in a positioning system for a mobile device comprising a flat housing and a flat display screen on the front side of the device. 
The present disclosure in particular, but not exclusively, relates to a first connecting element such as can be used with a positioning system for the device that was introduced on the market under the brand name iPad® by the Apple® company in the spring of 2010, or for a comparable device such as devices generally known as tablet pc's.  The control of such a device can to a significant extent take place via the display, which is of the touchscreen type.  Although such a device can be used as a separate device, for example while lying on a table or being held in a user's hand, it is advantageous in the case of some applications if the device is positioned in a particular space.  Such a space may be a room of a building but also, for example, the interior of a car.  Further, such an application may include the display of the mobile device to be used for displaying a film (e.g., during a car ride) or a recipe (e.g., in a kitchen environment).  In particular, because of the touch screen operation and also because of the larger dimensions and the greater weight of the above-mentioned mobile computing device in comparison to that of mobile telephones, there is a need for increased stability for a positioning system such as the iPad® and similar devices in comparison to positioning systems for smaller mobile telephones. 
Conventionally, a positioning system for an iPad® has been marketed under the name of ModulR®.  That system comprises a rectangular casing, on the inner side of which an iPad® can be clampingly accommodated.  On the outer side, the casing is provided with connecting legs near the corners thereof, which connecting legs are configured as projecting parts.  To enable mounting thereof to a wall, the ModulR system also comprises a wall element, whose outer circumference joins that of the casing, except at the location of the corners.  Holes are provided in the wall elements, via which the wall element can be screwed to a wall.  The wall element further comprises grooves into which the connecting legs can be moved.  The width of the groove decreases along part of the length, so that the connecting legs can be snapped in place in the grooves and the casing, with the iPad® clamped therein, can be attached to the wall element. 
Important drawbacks of that system include the fact that is quite visibly present on the wall when no casing carrying an iPad® is connected to the wall element which, for aesthetic reasons, will not be universally appreciated.  This is partially because of the wall element's dimensions.  In addition, the orientation in which the wall element is mounted on the wall also determines the orientation of the casing and thus of the display of the iPad®.  If a user opts for a landscape orientation of the wall element, it is not possible to position the display in portrait orientation on a wall. 
A drawback of the use of connecting legs on the rear side of the casing is that in the case of the iPad®, the connecting legs mar the design of the iPad®, which is very austere, without any projecting parts.  In general, it can be stated that most mobile devices do not have any parts that project to a substantial extent.  In variants of the ModulR system, an element such as the above-described wall element is attached to the end of a pivoted arm that can be fixed to a wall or to the rear side of a head restraint of a car seat.  Drawbacks comparable to those described above apply thereto.