http://linuxcnc.org/docs/html/gcode/gcode.html If your looking to hack your G-code and it does not do what you are wanting then this link will help. Extract 1. Conventions Conventions used in this section In the G code prototypes the hyphen (-) stands for a real value and (<>) denotes an optional item. If L- is written in a prototype the - will often be referred to as the L number, and so on for any other letter. In the G code prototypes the word axes stands for any axis as defined in your configuration. An optional value will be written like this <L->. A real value may be: An explicit number, 4 An expression, [2+2] A parameter value, #88 A unary function value, acos In most cases, if axis words are given (any or all of X Y Z A B C U V W), they specify a destination point. Axis numbers are in the currently active coordinate system, unless explicitly described as being in the absolute coordinate system. Where axis words are optional, any omitted axes will retain their original value. Any items in the G code prototypes not explicitly described as optional are required. The values following letters are often given as explicit numbers. Unless stated otherwise, the explicit numbers can be real values. For example, G10 L2 could equally well be written G[2*5] L[1+1]. If the value of parameter 100 were 2, G10 L#100 would also mean the same.