What Does CO Stand For?

What does CO stand for most often?

1. Colorado: the USPS code on envelopes and forms, as in Denver, CO 80202.
2. Company: the abbreviation Co. in names like Smith & Co. or Acme Supply Co.
3. Carbon monoxide: the gas measured by home detectors, written CO.
4. Commanding officer: the officer in charge of a military unit.
5. Care of: written c/o on mail sent through another person.

Other uses exist, such as conscientious objector, change order in construction and the ISO code for Colombia, but the five above cover most searches.

CO abbreviation by field

CO for Colorado

The US Postal Service assigned CO to Colorado, and it is the form to use on envelopes, shipping labels and online forms. Older writing and some style guides use the traditional abbreviation Colo., with a period, in running text. The Colorado abbreviation page explains when to use each.

Do not mix it up with CT for Connecticut or CA for California, two codes people sometimes type by mistake.

Co. for company

In business names, Co. is short for company and takes a period in American style: Johnson Paint Co. You may also see "& Co." after a list of partners, meaning "and company," or "and the rest of the firm." It shows up alongside other business endings such as Inc., LLC and Ltd. Unlike those, Co. alone does not describe a specific legal structure; it is simply part of the name.

In casual speech, "and co." also means "and friends": "Maya and co. are on their way."

CO for carbon monoxide

In chemistry, CO is the formula for carbon monoxide: one carbon atom and one oxygen atom. The gas has no color or smell and can be produced by furnaces, stoves, car engines and generators that burn fuel. That is why homes use CO detectors, often combined with smoke alarms.

Do not confuse it with CO2, carbon dioxide, which has two oxygen atoms and is the gas people breathe out. In the formula, the O is a capital letter and the C is too, so writing Co by mistake changes the meaning: Co is the chemical symbol for cobalt.

CO in the military

A CO is the commanding officer, the person in charge of a unit such as a company, a battalion or a ship. The second in command is often called the XO, the executive officer. In speech, service members say "the C O" or "the skipper" on a ship. For military ranks and their short forms, see captain and MOS.

Reading CO correctly

Ask where the letters appear. When it follows a town and comes right before the postal ZIP, it is Colorado. After a business name, it is company. On a detector, it is carbon monoxide. In an order or a base memo, it is a commanding officer. Say it as two letters, "see oh," in every case. The famous acronyms list has plenty more two and three letter puzzles like this one.