An Overview

The Party Line or as it is officially known, Shared Service, is a complex subject and changed over the years as technology progressed.

This is where more than one subscriber shared a line.  It was generally used to reduce costs on line plant and later exchange equipment.

Whilst the customer may have a reduced line rental, they complained about the inconvenience especially if the other party was using the phone a lot.

The first shared lines were on the old manual exchanges and the switchboard operators would have to ring the lines a number of times depending on how many were sharing.  In the US, where lines could reach 20 miles in length this was quite common, but it also allowed others to listen in as all the phones rang.  But, because many of them knew each other, they also arranged conference calls so they could all chat and get the local gossip.

In the UK some shared lines could have up to 10 subscribers on them, but only one person could be involved on a call.  As the line circuit consisted of two wires the sharers would be split so that half would use one wire and the other half the other wire.  But that was just for incoming ringing and the speech aspect would be shared by all.  To differentiate the telephones on a shared line of this type the 10 subscribers would be allocated a code.  The 5 telephones on one wire would be called X plus a number from 1 to 5.  Telephones on the other wire would be called Y plus a number from 1 to 5.  When requesting to be connected the caller would ask the switchboard operator, for example, 15X4 which is exchange number 15 and telephone 4 on the X wire.  The switchboard operator would then ring the X wire 4 times of number 15.  You can see these numbers in old telephone directories.

As Automatic systems were introduced then sharing caused a problem as the exchanges in the UK could only differentiate between the two wires and Party Lines then just consisted of two customers.  To make an outgoing call you just lifted your receiver and dialled.  Incoming calls could differentiate between the two as there were two wires, so the right phone would ring.  Because there was no differentiation on outgoing calls the two parties had to negotiate the bill!  That did not go down to well!

The automatic systems were later designed so that when an outgoing call was to be made a button was pressed to get dial tone.  This differentiated the two lines and it was called the Separate Metering System as each party had their own meter in the exchange.

This system continued to be in use until the late 1980's when System X and System Y exchanges were introduced into the UK and these systems did not provide for Shared Service.

But all was not lost as a Carrier system could be used in which two parties would share a line but did not know that they were sharing as no press button was required.  The Carrier system used a higher frequency for one party so they could not hear each other and each phone only rang if their line was called.  Not really Shared Service but in principle two people shared the circuit.

Click here - for a technical description of the separate metering system.



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Last revised: October 05, 2023