The names for the two wires of a telephone line were derived from the electrical contacts on the original 1/4 inch telephone plug. These plugs are similar in appearance to the commonly used 6.5mm diameter stereo phono plugs
One electrical contact is at the tip of the plug (used for Line +) and the other is a ring just above it (used for Line -). There is also a third contact referred to as the sleeve.
Conventional nomenclature is:
|TIP (note)||A-wire||Line (+) positive||Earth||Red|
|RING||B-wire||Line (-) negative||Battery||Black|
Note: A useful memory aid is to remember "Red TAPE" : (Red = Tip, A-wire, Positive, Earth