Convert between effective annual yield and bond-equivalent yield (semi-annual).
BEY is a convention that annualizes yields by simply doubling the semi-annual rate (no compounding). It's the standard way US bonds quote yields because most pay coupons semi-annually.
BEY slightly understates the true annual return because it doesn't compound the two semi-annual payments. A BEY of 5.5% has an EAY of 5.576%. The difference grows with higher rates.