The actual/actual interest accrual convention is recommended for eurodenominated bonds. There are at least three different interpretations of actual/actual. These three interpretations are identified as:
The difference between the ISDA, ISMA and AFB methods can be reduced to a consideration of the denominator to be used when calculating accrued interest. In all three cases, the numerator will be equal to the actual number of days from (and including) the last coupon payment date or period end date, to (but excluding) the current value date or period end date.
Under the Actual/Actual (ISDA) approach, the denominator varies depending on whether a portion of the relevant calculation period falls within a leap year. For the portion of the calculation period falling within a leap year, the denominator is 366, for the other portion the denominator is 365. The ISDA convention is also known as Actual/Actual (Historical), Actual/Actual, Act/Act, and according to ISDA also Actual/365, Act/365, and A/365.
Under the Actual/Actual (ISMA) approach, the denominator is the actual number of days in the coupon period multiplied by the number of coupon periods in the year. The ISMA and US Treasury convention is also known as Actual/Actual (Bond).
Under the Actual/Actual (AFB) approach, the denominator is either 365 if the calculation period does not contain February 29th, or 366 if the calculation period includes February 29th. The AFB convention is also known as actual/actual (Euro).
Consider some examples:
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First you will use a day counter that follows the ISDA convention.
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 (2.1.1) 
The numerator is equal to the actual number of days from (and including) the last coupon payment date or period end date, to (but excluding) the current value date or period end date. Therefore, the number of days from January 1st, 2006 to July 1st, 2006 can be calculated by adding the number of days in January, February, March, April, May, and June together:
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 (2.1.2) 
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 (2.1.3) 
The denominator for ISDA is 365 since the year of 2006 is not a leap year:
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 (2.1.4) 
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 (2.1.5) 
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 (2.1.6) 
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 (2.1.7) 
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 (2.1.8) 
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 (2.1.9) 
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 (2.1.10) 
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 (2.1.11) 
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 (2.1.12) 
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 (2.1.13) 
In the second example you will use the ISMA convention.
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 (2.1.14) 
As you can see the number of days between January 1st, 2006 and July 1st, 2006 is the same according to both conventions. However, the length of the period from January 1st, 2006 to July 1st, 2006 as a fraction of the year is different.
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 (2.1.15) 
The denominator is the actual number of days in the coupon period multiplied by the number of coupon periods in the year.
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 (2.1.16) 
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 (2.1.17) 
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 (2.1.18) 
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 (2.1.19) 
Finally, consider the AFB day counting convention.
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 (2.1.20) 
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 (2.1.21) 
The denominator is either 365 if the calculation period does not include February 29th, or 366 if the calculation period includes February 29th.
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 (2.1.22) 
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 (2.1.23) 
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 (2.1.24) 
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 (2.1.25) 
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 (2.1.26) 
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 (2.1.27) 
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 (2.1.28) 
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 (2.1.29) 
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 (2.1.30) 
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 (2.1.31) 
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 (2.1.32) 
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 (2.1.33) 