LinuxCNC G5 Cubic Spline

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.

Source checked19/19 values · 2026-10-26

Specification

1 source

Language	gcode
Code	G5
Syntax	G5 X- Y- <I- J-> 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

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.

Safety

Operating limit: the active plane must be G17, and only X and Y axis words are allowed; otherwise the command is an error.