25-pair color code

The major, or primary group of colors consists of the sequence of white, red, black, yellow, and violet (mnemonics Why Run Backwards, You'll Vomit).

[3] The minor, or secondary color is chosen from the sequence blue, orange, green, brown, and slate (mnemonics Bell Operators Give Better Service[4]).

[citation needed] When used for plain old telephone service (POTS), the first wire is known as the tip or A-leg (U.K.) conductor, and is usually connected to the positive side of a direct current (DC) circuit, while the second wire is known as the ring lead or B-leg (U.K.), and is connected to the negative side of the circuit.

The tip and ring convention is based on the 1⁄4″ (6.5 mm) TRS phone connectors, which were employed in telephone switchboards in the 19th and 20th centuries, where the tip contact of the connector is separated from the ring contact by a spacer of insulation.

A common application of the 25-pair color code is the cabling for the Registered Jack interface RJ21, which uses a female 50-pin miniature ribbon connector, as shown in the following table.

The pattern then starts over with the first 25-pair group as white/blue, and continues indefinitely, in multiples of 600 pairs or parts thereof.

There are indicators on the mylar ribbons to know where to begin for each layer and a diagram for the different cable sizes should be readily available for reference.

Older Bell System wiring inside customer premises used a cable type with four conductors of solid copper wire with the insulation colors red, green, yellow, and black (legacy) or a two-color code of blue, orange, and white.