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After playing Skyrim, while watching Game of Thrones, I realized that I do not understand the difference (if any) between "cog" and "gear".

A summary search on the Internet leads me to believe that they are the same thing, but I cannot be sure.

I have in mind two possibilities:

  1. "cog" and "gear" are true synonyms, they are interchangeable;
  2. a "gear" is a kind of a "cog"; with other words, all "gears" are "cogs", but not all "cogs" are "gears".
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  • Both words have several different meanings, some overlapping, some not. But probably the bit that interests you is that a cog usually means a single "tooth" on a gear wheel, whereas the noun gear usually refers to multiple interlocking cogged "drive" wheels, coupled in an arrangement designed to change the rotational speed between input and output.
    – FumbleFingers
    Commented Jan 17, 2020 at 13:27
  • This was asked a few days ago. Possible duplicate.
    – Michael Harvey
    Commented Jan 17, 2020 at 13:31
  • @FumbleFingersReinstateMonica: I feel that you have a point, but your definition of "cog" far beyond my abilities to imagine things :( Maybe use different words?
    – virolino
    Commented Jan 17, 2020 at 13:32
  • @MichaelHarvey: possible, but I was not able to find it.
    – virolino
    Commented Jan 17, 2020 at 13:32
  • A cog is a tooth is a sprocket - one of the "sticky-out bits" on the outside edge of a gear wheel that interlocks with a corresponding "notch" (hollow) on another gear wheel. People might sometimes refer to a single gear wheel as "a gear", but it's more often used as a kind of adjective (a gear wheel / assembly / box / ...). Or more abstractly, as in You start the car moving by using first gear.
    – FumbleFingers
    Commented Jan 17, 2020 at 13:36

3 Answers 3

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The word "gear" has lots of senses that are not related to cogs. You can't replace the word "gear" by "cog" in these examples:

I put the car in gear and moved off.

I've got my camping gear, like my tend and sleeping bag.

There are some idioms using "cog" to mean "cogwheel" or "gear wheel" in which you can't replace "cog" by "gear":

He is just a cog in the machine.

Moreover a "cog" or "cogwheel" refers to the shape, but "gear" links with the function. In the strict sense, a "cog" is a tooth on a wheel. A cogwheel is any wheel with teeth. A gear is a cogwheel used mesh with another cogwheel. And a sprocket is a cogwheel that links to another cogwheel by means of a chain. A car's transmission has several gears. A bicycle uses spockets and a chain.

In common use it sounds more natural to say "he decorated his hat with cogs" than ... with gears". You are referring to the shape of the wheels, and not their function. The word "gear" suggests a heavier wheel designed for power. You car's transmission has gears, but your watch uses cogs.

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Technically, I think the cog is actually one of the teeth on a gear.

Gear : a wheel having pointed parts around the edge that come together with similar parts of other wheels to control how much power from an engine goes to the moving parts of a machine

Cog : one of the tooth-like parts around the edge of a wheel in a machine that fits between those of a similar wheel, causing both wheels to move

Although I have heard people refer to a single wheel with teeth as a cog too, and seen it used in Elder Scrolls games (Dwemer cogs). This may or may not be incorrect. Cambs & OED state it's the teeth but I have found one example of it referencing both. I've also noted 'Cogged Wheel' in the OED definition of cog, and cog-wheel(OED).

Gear may also be restricted to a wheel with teeth(cogs) that is used to control the speed of the connecting parts (Among the plethora of other meanings)

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  • You might be right, but I remember (and an internet search confirms) that the Skyrim cogs are actually wheels with "teeth". I am aware that Skyrim might not be the best reference for linguistic or scientific accuracy.
    – virolino
    Commented Jan 17, 2020 at 13:36
  • @virolino Yes I've seen this too (they're called Dwemer cogs), but this is possibly incorrect (see updated answer)
    – Smock
    Commented Jan 17, 2020 at 16:27
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A gear follows the law of gearing. Cogs are driving dogs on a wheel. A cog wheel is a wheel with a series of cogs on in.

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