Why do so many coins use X__ as their abbreviation?

[Bizmark13]

Anyone else here noticed this trend?

For example, Monero is XMR, Ripple is XRP, and Primecoin is XPM. There doesn't seem to be any connection between these three coins other than the fact their abbreviations all begin with X__. Monero is a Bytecoin (BCN) relaunch, Ripple is an internal currency used in an ecosystem developed by a for-profit company founded by Jed McCaleb from Mt. Gox, and Primecoin is a coin developed by Sunny King who also created Peercoin (PPC).

Edit: It seems Crypti uses XCR too.

What is it about the letter X that makes it so popular?

[tonny.jr]

i think X stands for unknown,it's mean anything can happen

[Sukrim]

http://en.wikipedia.org/wiki/ISO_4217#X_currencies

[fluffypony]

It's for ISO 4217 compliance: http://en.wikipedia.org/wiki/ISO_4217#X_currencies

Bitcoin is officially XBT for the same reason: http://www.xe.com/currency/xbt-bitcoin

[Brewins]

n addition to codes for most active national currencies ISO 4217 provides codes for "supranational" currencies, procedural purposes, and several things which are "similar to" currencies:

    Codes for precious metals Gold (XAU), Silver (XAG), Palladium (XPD), and Platinum (XPT) are formed by prefixing the element's chemical symbol with the letter "X". These "currency units" are denominated as one troy ounce of the specified metal as opposed to "1 USD" or "1 EUR".

    The code XTS is reserved for use in testing.

    The code XXX is used to denote a "transaction" involving no currency.

    There are also codes specifying certain monetary instruments used in international finance, e.g. XDR is the symbol for special drawing right issued by the International Monetary Fund.

    The codes for most supranational currencies, such as the East Caribbean dollar, the CFP franc, the CFA franc BEAC, and the CFA franc BCEAO begin with an "X". The predecessor to the euro, the European Currency Unit (ECU), had the code XEU.

The use of an initial letter "X" for these purposes is facilitated by the ISO 3166 rule that no official country code beginning with X will ever be assigned. Because of this rule ISO 4217 can use X codes without risk of clashing with a future country code. ISO 3166 country codes beginning with "X" are used for private custom use (reserved), never for official codes. For instance, the ISO 3166-based NATO country codes (STANAG 1059, 9th edition) use "X" codes for imaginary exercise countries ranging from XXB for "Brownland" to XXR for "Redland", as well as for major commands such as XXE for SHAPE or XXS for SACLANT. Consequently, ISO 4217 can use "X" codes for non-country-specific currencies without risk of clashing with future country codes.

The inclusion of EU (denoting the European Union) in the ISO 3166-1 reserved codes list, allows the euro to be coded as EUR rather than assigned a code beginning with X even though it is a supranational currency.