Master Units
Most printers support commands with a variety of horizontal and vertical resolutions. For example, the Immediate Line Feed command for a particular printer might provide a resolution of 1/288th of an inch, while the same printer might support a vertical graphics resolution of 1/96th of an inch. Likewise, this printer might also provide horizontal resolutions of 1/80th, 1/160th, and 1/320th of an inch.
Unidrv provides a single coordinate system to handle these various resolutions. The units in this coordinate system are called master units. A printer's master units are expressed as an (x, y) pair of values, where x is the master unit for the horizontal direction and y is the master unit for the vertical direction.
To determine a plane's master units, calculate the least common multiple (LCM) of the denominators for the actual resolutions. Using the example printer, you would do the following:

Calculate the LCM of 80, 160, and 320, which is 320. Thus, the horizontal master unit is 1/320th of an inch.

Calculate the LCM of 288 and 96, which is 576. Thus, the vertical master unit is 1/576th of an inch.
Important Both of the master unit values and the vertical and horizontal resolutions should be a multiple of the number of pins in the print head (that is, the PinsPerPhysPass value). If this condition is not met, it is possible that extra blank lines will be produced for certain paper sizes.
To specify a printer's master units, use the *MasterUnits attribute. The attribute's format is as follows:
*MasterUnits: PAIR ( X_Denominator , Y_Denominator ) 
where X_Denominator is the LCM of the denominators for the horizontal resolutions, and Y_Denominator is the LCM of the denominators for the vertical resolutions. The following GPD entry specifies the master units for the example:
*MasterUnits: PAIR(320, 576)
Generally, position and size values used in GPD file entries must be specified in master units. For example, to specify that the maximum custom page size for our example printer is 9 inches by 12 inches, the following entry would be used, where 9x320=2880 and 12x576=6912:
*MaxSize: PAIR(2880, 6912)
When calculating values for master units, use only the device resolutions that you want Unidrv to support. For example, if a printer supports horizontal resolutions of 1/80th, 1/96th, 1/160th, and 1/320th of an inch, but you do not intend to specify the 1/96th of an inch resolution within your GPD file, do not include it in your LCM calculation.
If your printer supports cursor commands for moving the cursor position, then the values specified for the *XMoveUnit and *YMoveUnitcursor attributes must be included in master unit calculations. Suppose, for example, that a GPD file contains the following entries:
*XMoveUnit: 60 *YMoveUnit: 60
When calculating this printer's master units, 1/60th of an inch must be included in the horizontal and vertical master unit calculations.