# LinuxCNC G5 Cubic Spline Instructions · `G5 Cubic Spline` · verification: source checked G5 creates a cubic B-spline in the XY plane using only the X and Y axes. P and Q give the second control point as offsets from the end point and must both be specified. I and J give the first control point as offsets from the start point; on later G5 commands they can be omitted so the spline continues smoothly from the previous cubic. ## Specification | Property | Value | |---|---| | Language | gcode | | Code | G5 | | Syntax | G5 X- Y- P- Q- | | Dialect | LinuxCNC | ### Parameters | Name | Data type | Description | |---|---|---| | X | real | | | Y | real | | | I | real | X incremental offset from start point to first control point | | J | real | Y incremental offset from start point to first control point | | P | real | X incremental offset from end point to second control point | | Q | real | Y incremental offset from end point to second control point | ## Safety - Operating limit: the active plane must be G17, and only X and Y axis words are allowed; otherwise the command is an error. ## Study notes - For the first G5 command in a series, I and J must both be specified. - For subsequent G5 commands, either both I and J must be specified or neither. - If I and J are omitted on a subsequent G5 command, the starting direction matches the ending direction of the previous cubic, as if I and J are the negation of the previous P and Q. - P and Q must both be specified for every G5 command. - It is an error to specify an axis other than X or Y, or to use a plane other than G17. - Example initial spline: G90 G17; G0 X0 Y0; G5 I0 J3 P0 Q-3 X1 Y1. - Example subsequent spline: G5 P0 Q-3 X2 Y2. ## Sources 1. [G-Codes](https://linuxcnc.org/docs/html/gcode/g-code.html#gcode:g5) (accessed 2026-10-26) --- Source page: https://machineschema.com/instruction/gcode-g05 JSON: https://machineschema.com/instruction/gcode-g05.json License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite Machine Schema and the original sources.