[Paul_S]

The other posts pretty much covered the subject.

2 ½ axis is typically X & Y interpolation, Z positioning; Y & Z interpolaton, X positioning; and last and not least, Z & X interpolation and Y axis positioning.

And G17, G18, and G19 to switch between the 2 ½ axis modes.

G17 X & Y interpolation.

G18 Z & X interpolaton. (X & Z if the X axis sign of the moves are in reversed to the cartesian coordinate system rotation. As looking down on a lathe or looking toward Y+ from Y- front of a mill. The reverse G2 is CCW and G3 is CW so to the operator G2 looks CW and G3 looks CCW.)

G19 Y & Z interpolation.

3 axis is X, Y and Z interpolaton.

The 4th axis, A axis typically rotates around the X axis.
A 5th axis, B axis typically rotates around the Y axis.
A 6th axis, C axis typically rotates around the Z axis.

Depending on the control, the machine builder, and/or third party add ons. the 4 axis could be any combination rotations and axis words.

Some controls have the option of switching the G18 from ZX to XZ to accomodate the operator. G18 ZX is programmer preferred. G18 XZ is operator (non-programmer) preferred. The latter format is typically used in conversational programming where the operator must specifiy CW or CCW in an XZ move.