Apple Macintosh Performa 577 - Manual (Page 127)

Adjustments Geometry - 6
Horizontal Size
Note:
Due to video features
and timing differences
across the Apple line of
Macintosh computers, the
width of the raster/image
area on the Performa 550
screen may vary up to 3/16
inch at each side of the
screen. Perform the
horizontal size adjustment
to set the display to its
proper width.
All-White Screen