ISO 4217 defines 3-letter currency symbols:
Do currencies\' minor units (cent, pence
In the financial markets there's roughly two established industry standards.
The first one is really a case-by-case agreement, mostly enforced by exchanges that have their securities quote in the minor currency unit. This lead to:
Probably pioneered by Reuters and Bloomberg, the second standard is far more wide-spread and consistent. The agreement is to lowercase the third letter to denote the minor units.
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