IBM 3270 Manual page 104

2. Press the Size key (PF12).
The line at the bottom of your screen changes
to:
WSCTRL SETUP SIZE WINDOW=C SCREEN=O
3. Press the black Left Cursor key.
You will see the right border of the first
window appear along the right side of your
screen.
Chapter 9. Rearranging the Windows 9-7