AT&T MERLIN LEGEND Release 2.0 Analog Direct-Line Console Operator's Manual page 27

About Your Direct-Line Console
Using DSS Buttons
The MERLIN II System Display Console has a built-in DSS.
You use the DSS buttons for one-touch dialing of extensions
when making or transferring calls and for manual signaling of
calls.
DSS buttons have assigned numbers usually in consecutive
order, starting with the top left corner button. Each button
can have three numbers assigned to it. For example, the
following illustration shows 10, 50, 90 assigned to the first
button.
To find out about the numbers assigned to your DSS, or if
your console is not labeled with extension numbers, see your
system manager.
1-16 Line and DSS Buttons