This asks Windows for the capability of your output screen. The index specifies which attribute you are interested in, and it returns a number corresponding to that. Some useful indices are as follows:
0 : Device driver version
2 : Device classification
4 : Horizontal size in millimeters
6 : Vertical size in millimeters
8 : Horizontal width in pixels
10 : Vertical height in pixels
12 : Number of bits per pixel
14 : Number of planes
16 : Number of brushes the device has
18 : Number of pens the device has
20 : Number of markers the device has
22 : Number of fonts the device has
24 : Number of colors the device supports
26 : Size required for device descriptor
28 : Curve capabilities
30 : Line capabilities
32 : Polygonal capabilities
34 : Text capabilities
36 : Clipping capabilities
38 : Bitblt capabilities
40 : Length of the X leg
42 : Length of the Y leg
44 : Length of the hypotenuse
45 : Shading and blending caps
88 : Logical pixels/inch in X
90 : Logical pixels/inch in Y
104 : Number of entries in physical palette
106 : Number of reserved entries in palette
108 : Actual color resolution
116 : Current vertical refresh rate of the display device (for displays only) in Hz
117 : Horizontal width of entire desktop in pixels
118 : Vertical height of entire desktop in pixels
119 : Preferred blt alignment