gnucash/Docs/xacc-currency.html

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A double entry system stores both values (and makes teh transaciton balance):

double entry states that value1+value2+value3+ ... = 0
where each value is recorded in a different account.
A value is value=exch-rate*amount (for currency accounts)
           value=price*num-shares (for stocks/mutual-funds)

Thus, buying a widget in japan and using dollars to pay for it would appears
like so:

SpendAcct: (an oridnary bank/cash acct valued in USD)
----------
Date      xfer from       memo                cost
1/5/99    [YenAcct]       Buy widget          $1.00


YenAcct: (a currency trading account valued in yen)
--------
Date      xfer from       memo                purchase   rate
1/5/99    [SpendAcct]     Buy widget          Y150       0.00667


The engine links together all three of these values (1.0, 150, 0.00667)
permanently and makes it imposssible to change one without changing another,
so that the grand total is always zero.


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