COM jack
COM is the common jack: the black test lead’s socket, used for every measurement. It is the low, or negative, side of the meter’s input.
Easily confused with
How to read it
Fluke’s guide calls COM the black test lead input for all measurements, the low or negative connection for circuit measurements and accessories, also known as the return terminal; COM is short for common. Fluke’s 114, 115 and 117 manual lists it as the common (return) terminal for all measurements, and Extech’s EX330 manual calls it the negative COM jack.
The red lead’s jack changes with the setting. On Fluke’s 114 to 117 one jack takes voltage, continuity, resistance, capacitance, frequency and diode tests, and another takes current up to 10 A; Klein’s MM400 has 10A, COM and VΩ jacks. Fluke’s guide says the red lead’s jack has to match the setting on the dial, or the meter, the equipment and the technician are all at risk.
Also searched as: black lead jack, common terminal, COM on a multimeter, return terminal.
These pages describe what each setting, marking and reading means. They are not instructions for measuring anything. Meters differ between makers and models: the meter’s own manual says what each of its symbols means, which measurements it is rated for, and how to use it safely.
Look-alikes
The 10A and mA jacks are for current, and the V or VΩ jack for voltage and most other settings. COM is the one jack every setting uses.
The limit on COM
Extech’s safety rules for the EX330 say not to measure voltage if the voltage on the COM jack is more than 600 V above earth ground.
Read next
- Next in Multimeter symbols10A fused jackThe 10A jack is the input for currents up to 10 amps, and FUSED means a fuse inside the meter protects it. A separate mA or µA jack, also fused, takes small currents.
- The same shape in AI labels and iconsAI iconAI-generated or AI-changed content. A badge with the letters AI, often in a circle or beside a word such as generated or modified, labels an image, video, sound or text as made or altered with artificial intelligence. The European Union publishes an official set of these icons.
More multimeter symbols
The same shape elsewhere
Browse symbols drawn with circles or letters and numbers.
Sources
- Multimeter Symbols, Buttons, Dials, and Display, Fluke learning centre article, Fluke Corporation, Digital multimeter jacks
- 114, 115, and 117 True-rms Multimeters: Users Manual, PN 2572573, July 2006, Rev. 1, 2/07, Fluke Corporation (copy hosted by UCL Electronic and Electrical Engineering), Terminals
- MM400 Auto-Ranging Digital Multimeter: Instruction Manual, MM400 instruction manual, Klein Tools (copy hosted by UBC Earth, Ocean and Atmospheric Sciences), Feature details
- Mini Multimeter with Non-Contact Voltage Detector, Model EX330: User’s Guide, EX330-EU-ENG V4.2 1/08, Extech Instruments (copy hosted by Core Electronics), Controls and jacks; Safety instructions
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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