The Numbering system defines the number of speakers present in the system. The First number denotes the number of full range Audio Channels, those that are able to reproduce the higher frequencies, where most all the detail and audio information is produced. the last number declares the number of low frequency "Sub Woofer channels" that are used.
In a 5.1 surround system there should be 5 separate speaker enclosures, each attached to the audio source by separate wires on unique audio channels, PLUS 1 sub woofer channel.
A 2.1 surround system would have a more traditional "Stereo" arrangement with a single sub woofer channel.
It is possible to attach more than one speaker to a single audio channel, this does not change the numbering designation, because there is no change in the number of separate sound channels.
FM 21-11 is numbered in the old system. FM 4-25.11 is numbered in the new system. The book is the same.
-21
47
21
The Sound of the Jam was created on 2002-05-21.
The Sound Inside was created on 2005-03-21.
89 - 21 = 68
It is 55 minus 21 = 34
The common difference is 13 because 21-8=13, 34-21=13, and 34-47=13.
West Sound FM was created on 1990-05-21.
10% difference.
4