This excerpt taken from the BP 6-K filed Mar 13, 2006.
Held-to-maturity investments Non-derivative financial assets with fixed or determinable payments and fixed maturity are classified as held-to-maturity when the Group has the positive intention and ability to hold to maturity. Held-to-maturity investments are carried at amortized cost using the effective interest method. Gains and losses are recognized in income when the investments are derecognized or impaired, as well as through the amortization process. Investments intended to be held for an undefined period are not included in this classification.