Why Three Wires in a Telephone Line?

http://www.cisco.com/networkers/presentations/voice/0948_038.ppt

Thank you for your PPT.

I still cannot understand why I have 3 wires. From viewing, all the three wires are regular AWG 16 wire, neither sheild wire nor grounding (GREEN) wire. :(

Without knowing what system you're working with, I can only guess that the third wire is used by the PBX.
... and 16 AWG for telephone wire??

Back in the old days of party lines, the yellow wire was used for ground, which was used by party line ringers.The red and green were for ring and tip of dial tone/voice.

Only 2 wires are now used, but the 3 conductor wire is still being made.Sometimes you may see 4 wires,where there are 2 separate services(outside lines, or pbx lines) are required.These will have an additional wire, a black wire.So pair #1 will be red/green, and pair 2 will be yellow black.

So basically, you can ignore the yellow wire, and keep it in mind in case one of the others goes bad.

I need to learn more about telephone. Thank you so much for above replies.