Why 4 pins for the speaker connection on motherboards?

[Stephen Tashi]

For as long as I can remember, the typical speaker connection on computer motherboards has 4 pins and the typical speaker has a 4 slot female connector with only two wires leading to the speaker. Is a there a technical or historical reason for the 4 pin connector?

 

[davenn]

For as long as I can remember, the typical speaker connection on computer motherboards has 4 pins and the typical speaker has a 4 slot female connector with only two wires leading to the speaker. Is a there a technical or historical reason for the 4 pin connector?

do they ?

All my computer (over many years) speaker/headphone connector is 3 contact ... ie... stereo ... Left, Right, Common

maybe a pic of what you are referring to may help clarify ?

 

[Borek]

 

[trurle]

Inner lines in original PC 4-pin speaker connector are the "key" pin and ground pin, preventing connector from been connected in reverse polarity to active (amplifier-containing) speakers. With the typical passive (symmetric) speakers it is a useless feature, therefore inner lines are not used in the picture you shown.