Currency codes are maintained under ISO 4217 by SIX, which describes itself as the only recognised authoritative source on currency code designations. When a currency is replaced, its entry moves to a historic list with a withdrawal date, and a new entry appears with a new three-letter code and a new three-digit numeric code. Mauritania's old ouguiya, MRO, numeric 478, carries a withdrawal date of December 2017; the current ouguiya is MRU, numeric 929. The International Monetary Fund records the redenomination itself at a rate of one to ten in January 2018.
When a country replaces its currency, most of the attention goes to the notes. The part that decides whether the change reaches a bank in another country is smaller and duller: a row in a register, and two codes rather than one.
Key Takeaways
- Currency codes are maintained under ISO 4217, and SIX describes itself as the only recognised authoritative source for them.
- A replaced currency moves to a historic list with a withdrawal date; a new entry appears with a new letter code and a new numeric code.
- Mauritania's MRO, numeric 478, carries a withdrawal date of December 2017. The current ouguiya is MRU, numeric 929.
- Where a country replaces its own currency the two-letter country prefix persists. Where it adopts another country's, it does not.
- The register says what a code means now. It cannot reach a system that copied the old list.
The register, and who keeps it
One list, with a named custodian.
Three-letter currency codes are defined by ISO 4217, and the maintenance agency for them is SIX, which states that it is the official maintenance agency of these currency codes under ISO 4217 and as such the only recognised, authoritative source on currency code designations. It publishes three things: the current list, a historic list of codes that have been withdrawn, and an overview of amendments. It describes changes to the lists as occurring if a currency is created, withdrawn, or parts of it are altered.
That structure is the whole mechanism. A currency does not disappear from the record when it is replaced; it moves, and it takes a withdrawal date with it.
What a replacement looks like in the register
Two entries, and four values that change.
Mauritania is a clean example because the change was recent and uncomplicated. The historic list carries MAURITANIA, Ouguiya, MRO, numeric 478, with a withdrawal date of 2017-12. The current list carries MAURITANIA, Ouguiya, MRU, numeric 929.
Note that the currency's name did not change. It is the ouguiya in both rows. What changed is the code, the number, and the value of the unit: the International Monetary Fund records the change of currency and its redenomination at a rate of one to ten as taking place in January 2018.
There is a small mismatch worth noticing between those two dates. The register's withdrawal date is December 2017, and the new money arrived in January 2018. The register is recording when the old code ceased to be current rather than when the last old note left a pocket, and those are different events.
Why the numeric code changes completely
Because the two codes are constructed on different principles.
A three-letter code opens with the ISO two-letter country code. The third letter is chosen when the code is assigned and is not systematic: the N in RON stands for new, the G in ZWG for gold, and the U in MRU for nothing in particular. So the informative part is the prefix, and it persists whenever a country is replacing its own currency with another of its own.
| Old | Withdrawn | New | What the letters do |
|---|---|---|---|
| MRO, Mauritania, numeric 478 | December 2017 | MRU, numeric 929 | Country prefix kept, last letter changed |
| MGF, Malagasy franc, numeric 450 | December 2004 | MGA, Malagasy ariary | Country prefix kept |
| ROL, Romanian old leu, numeric 642 | June 2005 | RON | Country prefix kept |
| ZWL, Zimbabwe dollar, numeric 932 | September 2024 | ZWG | Country prefix kept |
| ANG, Netherlands Antillean guilder, numeric 532 | March 2025 for Curacao and Sint Maarten | XCG | Prefix abandoned, see below |
| DEM, FRF, ITL, NLG, PTE, GRD and others | Through the euro accessions | EUR | Prefix abandoned, the country is no longer the issuer |
The numeric code has no such structure at all. It is assigned from a pool, so 478 becomes 929 and nothing connects them. That produces a practical asymmetry in the cases where the prefix does persist: a system keyed on the letters can often see that MRO and MRU are the same country's money, while a system keyed on the number sees two entirely unrelated currencies.
The prefix does not always survive, and the historic list makes clear when it does not. The largest set is the euro accessions: DEM, FRF, ITL, NLG, PTE, GRD and the rest were all withdrawn in favour of EUR, and a country that adopts another currency has no prefix to keep. The Caribbean case is the second kind. Codes beginning with X are used where a currency is not the currency of a single country, and the Caribbean guilder serves both Curacao and Sint Maarten, so a country-prefixed code was not available. The register shows the consequence plainly: ANG appears in the historic list twice, once for the NETHERLANDS ANTILLES with a withdrawal date of October 2010 and again for CURACAO and for SINT MAARTEN with a withdrawal date of March 2025. One code, retired twice, for two different reasons, fifteen years apart.
What the register cannot do
Reach anybody's software.
This is the practical half, and it is why an old code turns up long after the currency has gone. The register is authoritative about what a code means today. It has no way of correcting a value that was copied into a database, a spreadsheet or an integration years ago, and no way of knowing that anything did the copying.
So a withdrawn code has a long tail. It survives in historic records that are correct to keep as they are, and in current systems where it is simply wrong. A spot rate stored against a withdrawn code is a value with nothing on the other side of it. Nothing about the withdrawal distinguishes the two cases, which is exactly why the historic list is published: it is the only way to establish that a code was real, what it meant, and when it stopped being current.
A currency pair quoted with a withdrawn code is not a small formatting issue. It names money that no longer exists, and the arithmetic behind it is off by whatever the replacement ratio was, which in Mauritania's case is a factor of ten.
What a code change does not tell you
Almost nothing, and that is the point worth keeping.
A new code records that a replacement happened. It does not record why, or at what ratio, or what happened to holders of the old notes. Those live with the issuing authority: the Banque Centrale de Mauritanie describes itself as the monetary authority responsible for the issue, management and regulation of the notes and coins in circulation, and it names the national currency as the ouguiya, MRU.
The register and the central bank answer different questions. One tells a payment system what to call the money. The other decides what the money is. Reading the first as though it were the second is how a replacement gets described as a devaluation, which it need not be at all: a redenomination changes how many digits it takes to say an amount and leaves the value alone.
The practical consequence is short and it is about what to distrust. The Australian dollar to ouguiya pair uses MRU, because that is what the money is. A quote in MRO is not merely old formatting: it names a currency the register withdrew in December 2017, and if the underlying figure was never converted it is out by a factor of ten.
Where to look next
who maintains the codes on a transfer covers the other identifiers a payment carries. For the live figures, see the AUD to MRU page.
Sources
- SIX, Data standards: ISO 4217 currency codes - its statement that it is the official maintenance agency of the currency codes under ISO 4217 and the only recognised authoritative source on currency code designations, and that changes occur if a currency is created, withdrawn or parts of it altered
- SIX, ISO 4217 current currency and funds code list, published 1 January 2026 - Mauritania's current entry as the ouguiya, MRU, numeric 929
- SIX, ISO 4217 historic denominations list, published 1 January 2026 - MRO numeric 478 with a withdrawal date of December 2017, MGF numeric 450 withdrawn December 2004, ROL numeric 642 withdrawn June 2005, ZWL numeric 932 withdrawn September 2024, and ANG numeric 532 withdrawn in October 2010 for the Netherlands Antilles and in March 2025 for Curacao and for Sint Maarten
- International Monetary Fund, Islamic Republic of Mauritania: Selected Issues, Country Report Volume 2023 Issue 074 - the change of currency and its redenomination at a rate of one to ten in January 2018
- Banque Centrale de Mauritanie, Emission de billets et pieces - the bank as the monetary authority responsible for the issue, management and regulation of notes and coins, and the national currency named as the ouguiya, MRU
General information about currency codes and how currency replacements are recorded. Not financial advice, and not a recommendation of any product or institution. Reviewed by the Tradergy editorial team, September 2026.
Common questions
Who decides currency codes?
They are defined by ISO 4217, and SIX is the maintenance agency for them. In its own words it is the official maintenance agency of these currency codes under ISO 4217 and as such the only recognised, authoritative source on currency code designations. It publishes a current list, a historic list of withdrawn codes and an overview of amendments, and it states that changes occur if a currency is created, withdrawn, or parts of it altered.
What happened to MRO?
It was withdrawn. The historic list carries Mauritania's old ouguiya as MRO, numeric code 478, with a withdrawal date of December 2017. The current list carries the ouguiya as MRU, numeric code 929. The International Monetary Fund records the change of currency and its redenomination at a rate of one to ten as occurring in January 2018, so the register moved a month ahead of the notes.
Why does the numeric code change completely?
Because the two codes are built differently. The three-letter code opens with the ISO two-letter country code, so where a country replaces its own currency with another of its own the prefix persists and only the third letter moves: MRO to MRU, ROL to RON, ZWL to ZWG, MGF to MGA. The numeric code is assigned from a pool and carries no such structure, so 478 becomes 929 with nothing linking them.
When does the country prefix not survive?
Whenever the new currency is not that country's own. The largest set in the historic list is the euro accessions, where DEM, FRF, ITL, NLG, PTE, GRD and the rest were all replaced by EUR, which carries no country prefix at all. The other case is a currency serving more than one country: when the Netherlands Antillean guilder was replaced for Curacao and Sint Maarten the new code was XCG, because codes beginning with X are used where a currency does not belong to a single country.
Why do old codes still show up?
Because withdrawal from the register does not reach into every system that copied the list. The register is authoritative about what a code means today, not about what software is doing with a value stored years ago. That is why an old code can persist in an internal record, a spreadsheet or an integration long after the currency it named stopped existing, and why the historic list exists at all.