This feature is used in EU countries where the conversion to the EURO is being undertaken whilst the national currency is phased out. Basically the triangulation rule is that you have to convert from one participating currency to another, you must convert via the euro and round the euro amount to not less than three decimal places. So rather than a 1:1 currency it goes through the EURO. So say for example Euro convert to the secondary currency and then convert to the based currency, Euro X 2nd currency X base currency.
By default, the EUR currency is selected as the Triangulation currency. Only one currency can represent the Triangulation currency. To change the Triangulation currency, first clear the Reference currency for triangulation check box on the EUR record in the Currencies form. Then select the Reference currency for triangulation on the desired currency. Refer the Currencies form to enable Triangulation currency feature;
General Ledger -> Setup -> Currency -> Currencies