In a few documents about the FM transpon ... / 2014-08-21

In a few documents about the FM transponder on the SO-50 I noticed the use of the term PL tone where I expected the term CTCSS, for example in Operating SO-50 by Howard Long, G6LVB. I started wondering about the origins of PL tone and read the explanation on Continuous Tone-Coded Squelch System - en.wikipedia.org:

CTCSS is often called PL tone (for Private Line, a trademark of Motorola)

In amateur radio, the terms PL tone, PL and simply tone are still used somewhat commonly.

I'll keep calling it CTCSS.