ITU-T M.320 PDF

General

The position of a channel within a group is identified by a number starting from 1, the numbers of the different channels being taken in order of frequency in the basic group frequency band.

A channel is said to be erect within a group when the frequencies in the group-frequency band corresponding to the audio-frequencies in the channels ascend in the same relative order as those in the channels forming the group.

Similarly, a channel is said to be inverted within a group when the frequencies in the group-frequency band descend in the same relative order as the ascending order of the frequencies in the channels.

A group, supergroup, etc., is said to be erect when all of its channels are erect and is said to be inverted when all of its channels are inverted.