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Coin Collecting - The Basics of Proofs and Errors

You might call "proofs" and "errors" two-sides of the same coin...sometimes. They have one thing in common, they are sort of "rare", and this makes them valuable to coin collectors.

A "proof" is a coin which has been made to be a prototype, generally in order to test a design. Proofs are made in limited quantities and so, usually much later, they become highly valuable. Since they are usually as near to being pristine as a coin can be, their very condition makes them worth more than when they were new.

Sometimes, however, the proofing process goes awry. When something goes wrong, we generally call that an error, and coin collecting is no different. In fact, they call these errors...well..."errors".

Errors are coins that have been minted with some kind of flaw. Because of the mechanics of minting coins and proofs, several types of errors are relatively common.


Double-striking is a common minting technique used to make proofs that have very prominent features, but unless the registration, or positioning, is perfect double-striking can result in a 'shadow' on the finished coin.


Blank, or planchet, errors occur when the metal disk intended for striking (the planchet) remains blank after the strike. There are two types, one in which the rim has not yet been raised on the coin, the other when it has. Even though the coin has no currency value, it can still be prized by collectors.

As opposed to blanks, planchets are sometimes struck with the wrong denomination. A blank of the size and composition used for a dime might be struck with the look of a nickel, for example. Some U.S. denominations have even been struck on foreign planchets. Because of the rarity of such events, they're considered very valuable.


A "brockage" error occurs when the obverse (the 'face') and reverse ('back') of the coin are struck with incorrect images. For example, the obverse design might be struck on the reverse of the coin.

At times, the obverse image of one die and the reverse image of another die can wind up on the same planchet. These are called 'mule' coins, with one of the more famous examples being the Sacagawea Dollar/Washington Quarter. These are rare collectibles and much sought after by collectors.

There are several different types of die error which can occur. A 'die' is the piece, either top or bottom, that is etched or engraved with the negative image that will produce a positive image on the coin. The planchet, or blank, lies between two die and is struck to impress the images.


Like any metal piece, dies can be defective. A crack can open in the die, yet remain undetected. Coins struck with a cracked die show a distinctive mark. Some Morgan Dollars series are well-known for showing die cracks.


Collar die errors occur when the circular die above the anvil (the lower) die malfunctions.


Off-center coins occur when the planchet doesn't sit properly in the press. When struck, the image is off-center. These are relatively rare, and hence sometimes valuable...especially since modern mints go to so much trouble to catch such errors.


Mintmark errors result when the letter designating one mint (P for Philadelphia, for example) is struck onto a coin minted in another city (such as Carson City CC). This error can increase the value of what is otherwise an ordinary coin.

Though modern mints have extraordinary quality control, in earlier eras, however, processes had not yet developed to their current state. Therefore, most such errors are going to be found among older coins, which are generally already valuable in part because of their age. If you DO find a newer coin with an error, have it checked to ensure it's not a fake, and then start looking for collector who might be willing to pay big bucks for it.

Coin Supply Express
Collectible Coins - Proofs and Errors
Page Updated 4:01 PM Sunday 3/23/2014