The SetMapMode function sets the mapping mode of the specified device context. The mapping mode defines the unit of measure used to transform page-space units into device-space units, and also defines the orientation of the device's x and y axes. 

int SetMapMode(

    HDC hdc,	// handle of device context 
    int fnMapMode 	// new mapping mode 
   );	
 

Parameters

hdc

Identifies the device context. 

fnMapMode

Specifies the new mapping mode. It can be any one of the following values: 

Value	Description
MM_ANISOTROPIC	Logical units are mapped to arbitrary units with arbitrarily scaled axes. Use the SetWindowExtEx and SetViewportExtEx functions to specify the units, orientation, and scaling that you want.
MM_HIENGLISH	Each logical unit is mapped to 0.001 inch. Positive x is to the right; positive y is up.
MM_HIMETRIC	Each logical unit is mapped to 0.01 millimeter. Positive x is to the right; positive y is up.
MM_ISOTROPIC	Logical units are mapped to arbitrary units with equally scaled axes; that is, one unit along the x-axis is equal to one unit along the y-axis. Use the SetWindowExtEx and SetViewportExtEx functions to specify the units and the orientation of the axes that you want. Graphics device interface (GDI) makes adjustments as necessary to ensure the x and y units remain the same size (for example, if you set the window extent, the viewport will be adjusted to keep the units isotropic).
MM_LOENGLISH	Each logical unit is mapped to 0.01 inch. Positive x is to the right; positive y is up.
MM_LOMETRIC	Each logical unit is mapped to 0.1 millimeter. Positive x is to the right; positive y is up.
MM_TEXT	Each logical unit is mapped to one device pixel. Positive x is to the right; positive y is down.
MM_TWIPS	Each logical unit is mapped to one twentieth of a printer's point ( 1 / 1440  inch, also called a  "twip" ). Positive x is to the right; positive y is up.
 

Return Values

If the function succeeds, the return value identifies the previous mapping mode.
If the function fails, the return value is zero. 

Remarks

The MM_TEXT mode allows applications to work in device pixels, whose size varies from device to device. 
The MM_HIENGLISH, MM_HIMETRIC, MM_LOENGLISH, MM_LOMETRIC, and MM_TWIPS modes are useful for applications drawing in physically meaningful units (such as inches or millimeters). 
The MM_ISOTROPIC mode ensures a  1 : 1  aspect ratio. 
The MM_ANISOTROPIC mode allows the x-coordinates and y-coordinates to be adjusted independently.