Definition
The probe and nozzle are rarely at the same physical height. Firmware needs their measured relationship to turn a probe trigger into a nozzle-to-bed reference. A wrong value can place the nozzle too high for adhesion or too low, risking bed contact.
The sign and storage field are not universal: Marlin uses M851, Klipper calibrates probe z_offset, and RepRapFirmware prefers G31 while offering M851 compatibility with a sign conversion. Mechanical changes to the hotend, nozzle, probe or gantry can require recalibration.
What matters in practice
A physical measurement
The value belongs to one actual nozzle-probe installation, not a generic printer model.
Sign is dialect-specific
Read the target firmware's convention before entering or transferring a value.
First-layer critical
The offset works with homing and leveling data to establish nozzle height; it does not replace mesh or tramming.
Example in G-code
Marlin-only example
; Marlin example only
M851 Z-2.0
M500M851 sets a sample Z value and M500 asks supported Marlin configurations to save it. The number must be measured, not copied.
What this viewer can show
The viewer can show M851 in source but cannot measure the physical offset, apply hidden firmware compensation or certify first-layer clearance. Only explicit path coordinates are rendered.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.