Volume 

Menu: Geometry->Create->Contact volume

Contact volumes are defined between two surfaces that are physically in the same place but with different surfaces, lines and points. From a contact volume, it is possible to generate contact elements, to be used by some calculation algorithms, which define special contact between two bodies.

Those equivalent surfaces can be in the same location or can be separated by a movement (separated contact volume). The result will be equal meshes, ensuring a one-to-one relationship between nodes.

Choose 'contact volume' from the menu, and then select the surfaces. GiD automatically searches for possible contacts, combining the selected surfaces in pairs.

Contact elements are, by default, 8-node hexahedra or 6-node prisms (depending on the surface mesh).
However, you can select 2-node lines for all cases (see Element type).

The result elements can be also quadratic.

You can also select no mesh for the contact entity. This makes it possible to have exactly the same mesh for both surfaces but without any additional element between them.
When creating contact volumes, GiD internally checks what surfaces occupy the same location in the space and creates the contact, therefore there is no need to specify what surfaces have to be in contact. For this reason, several surfaces can be selected at once and GiD performs the contact automatically, indicating the number of contact volumes that have been generated.