IBM PCjr Manual page 79

Alternate Mode
In the Alternate mode, some keys will display different
symbols when used in combination with the Alternate
(Alt) key.
Notice that the Alt key has a blue border. All of the
keys used with the Alt key are marked with a blue
stripe underneath them. The symbols displayed by
using these keys with the Alt key are shown in the blue
stripes below the keys.
Keyboard 3-19