G90 vs G91: Absolute and Incremental Positioning

G90 and G91 set the distance mode — how the control interprets every coordinate word that follows:

• G90 absolute: coordinates are positions relative to the active work zero (your G54–G59 offset). G1 X10 goes to X=10.
• G91 incremental: coordinates are deltas from wherever the tool is now. G1 X10 moves by 10.

Both are modal and mutually exclusive; the active one governs all axis words until the other appears. The same four lines mean two different shapes depending on the mode:

; G90 absolute            ; G91 incremental
G0 X10 Y10                ; → point (10,10)          ; → 10 right, 10 up from current
G1 X20                    ; → to X=20 (moves 10)     ; → 20 further right
G1 Y0                     ; → down to Y=0            ; → 0 = no move at all!

That last line is the story of the two modes in miniature: in absolute mode Y0 is a real destination; in incremental mode it's a no-op.

#Why programs are posted in G90

An absolute program is restartable. If a cut stops on line 500, every line after it still describes a true position — jog clear, cycle-start from line 501, and the tool goes to the right place. An incremental program's meaning depends on flawless execution of every previous line; one skipped block shifts the entire rest of the part.

Incremental mode earns its keep inside repeated patterns: a subprogram that drills one pocket in G91 can be called at ten locations, doing the same relative dance at each. The pattern ends by restoring G90 — always.

G90 G0 X25 Y40    ; absolute: position at pocket 1
M98 P200          ; subprogram runs in G91 internally
G90 G0 X75 Y40    ; back to absolute for pocket 2
M98 P200

#G90.1 / G91.1 — the arc-center footnote

Distance mode covers axis words, but arc centers (I/J/K) have their own setting on controls that support it: G91.1 (default — centers are offsets from the arc start) and G90.1 (centers are absolute coordinates). CAM posts set it explicitly when the control supports it; hand-written code should leave it at the default. A program assuming one convention and a control set to the other is the classic cause of "radius mismatch" alarms on arcs that look perfect in the file.

#Common mistakes

• A stray G91 that never gets cancelled. The single most destructive modal mistake in G-code: everything after it re-interprets as deltas, and the "move to safe Z" at the top of the next operation becomes "move up a little from wherever you are." Preambles state G90 for exactly this reason.
• Assuming G91 in a subprogram restores itself. Modal state persists across subprogram returns on most controls. If a subprogram switches to G91, it must switch back — or the caller must.
• Testing an incremental file from the wrong start point. A G91 program is only correct from its intended start position. Simulate it and look at where the toolpath actually lands before trusting it.