Master chief petty officer

Master chief petty officer (MCPO) was an enlisted naval rate, a type of military rank used by the service organizations of different cultures. As a traditional naval grade, MCPO was the most senior grade of chief petty officer, and was almost the highest non-commissioned officer grade in organizations like Starfleet and the United States Navy. In comparison to infantry ranking systems, this rate was approximately equivalent to the most senior sergeant grade, such as sergeant-major. Personnel of this grade could be addressed simply as chief (or more properly, "master chief"), and sometimes by their job title, such as "master chief specialist."

While nominally subordinate to all commissioned officers, master chiefs wielded far more influence and authority than most junior officers and acted as mentors and advisors to senior officers. Master chiefs had decades of practical experience and were an indispensable source of technical skill and leadership on Starfleet vessels.

When there were individual grades of chief petty officers, senior chief petty officer (SCPO) was subordinate to MCPO.

In 2286, when a probe projecting humpback whale signals threatened to destroy Earth, Master CPO Janice Rand was one of the officers in Admiral Cartwright's command area. 

Background information
The rank of "master chief petty officer" was never referenced in dialog, but is rather derived from rank insignia.

The Star Trek movie-era rank insignia, designed by Robert Fletcher, was the first and only Trek insignia systems to feature separate pins for several realistic grades of CPOs. It is odd that Rand was in a chief uniform – while she was an enlisted chief in, when she appeared again in she was wearing officer insignia before being seen with MCPO insignia in Star Trek IV. Since the character in Star Trek III was simply credited as "woman", it may not necessarily have been Rand, but a different character.

Similar confusion in Rand holding various officer ranks dates back to the Marvel TOS "Eclipse of Reason" and DC TOS volume 2 "The Dream Walkers" comics, and continues in and.

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