Bell System - Carrier Systems

Carrier	Voice Channels†	Media	Repeater Spacing	Mode‡	First Used	Comments
A	4	Open Wire		Half Duplex	1918
B	3	Open Wire		Full Duplex	1920
C	3	Open Wire		Full Duplex	1924	Implemented as CN, CS, CT, and CU carriers. The second letter designates the exact frequency mappings used for the channels. This was needed to avoid crosstalk between lines on the same poles.
D	1	Open Wire		Full Duplex	1926
E	1	Commercial power lines		Full Duplex	1928	Never used commercially.
F						This designation was not used.
G	1	Open Wire		Half Duplex	1936
H	1	Open Wire		Full Duplex	1937
I						No data yet available.
J	12	Open Wire	50 miles	Full Duplex		Similar to CCITT/ITU G.311.
K	12	Cable	17 miles (using 19-gauge wire)	Full Duplex?
L	Varies	Coaxial Cable	Varies	Full Duplex
M
N	12 or 24	Cable
O	2-16?	Open Wire			1952?	See this article.

† On open wire media the number of channels is in addition to the single channel already present on the line for carriers A through H.
‡ The source uses the terms 2-Wire and 4-Wire for carriers A through H.

Sources:
A History of Science and Technology in the Bell System
Telecommunication Transmission Handbook