The disclosure relates to a holding frame into which plug connector modules can be inserted. The holding frame has guiding means, whereby the holding frame can be aligned with a second holding frame in the plug-in direction so as to fit in a precise manner. Two holding frames which can be plugged together contain at least one respective pneumatic plug connector module. The first holding frame comprises a first pneumatic plug connector module, and the second holding frame comprises a second pneumatic plug connector module. The first pneumatic plug connector module is aligned with the second pneumatic plug connector module such that the modules can be plugged together. Such pneumatic plug connector modules are particularly sensitive to wear when they are plugged together without being aligned relative to one another in an exact manner.