The most common application of the twisted pair is the telephone system.
Nearly all telephones are connected to the telephone company (telco) office by a
twisted pair. Both telephone calls and ADSL Internet access run over these lines.
Twisted pairs can run several kilometers without amplification, but for longer distances
the signal becomes too attenuated and repeaters are needed. When many
twisted pairs run in parallel for a substantial distance, such as all the wires coming
from an apartment building to the telephone company office, they are bundled together
and encased in a protective sheath. The pairs in these bundles would interfere
with one another if it were not for the twisting. In parts of the world where
telephone lines run on poles above ground, it is common to see bundles several
centimeters in diameter.