In computer networking terms there are many protocols and functionality in obselete / old data communication network is coined as x.something like the structure of a PAD is defined by the ITU-T in recommendations X.3, X.28, and X.29.
Sometimes, this is referred to as a Triple-X PAD, due to the three X series recommendations which define it.
In detail:
X.3 specifies the parameters for terminal-handling functions such as line speed, flow control, character echo, et al. for a connection to an X.25 host. The X.3 parameters are similar in function to present day Telnet options.
X.28 defines the DTE-C (asynchronous character mode) interface to a PAD, including the commands for making and clearing down connections, and manipulating the X.3 parameters. The commands were very crude, a bit like (but not at all compatible with) Hayes modem commands. Many commercial PAD products provided completely different enhanced user interfaces.
X.29 defines the DTE-P (packet mode) interface to a PAD, i.e. how the PAD encapsulates characters and control information in X.25 packets.
However Connections are established using X.121 and 14-digit X.25 addresses.
So now you can say penta-x referring to x.3, x.25, x.28, x.29. x.121