The location of the collateral assets should also be regulated to ensure the enforcement of investors’ rights. It is also important for Member States to lay down rules on the composition of the cover pool. Furthermore, coverage requirements should be specified in this Directive, without prejudice to the right of Member States to allow different means of mitigating risks such as with regard to currencies and interest rates. The calculation of the coverage and the conditions under which derivative contracts can be included in the cover pool should also be defined to ensure that cover pools are subject to common high-quality standards across the Union. The calculation of coverage should follow the nominal principle for the principal. Member States should be able to use a method of calculation other than the nominal principle, provided that the other method is more prudent, namely it does not result in a higher coverage ratio, where the cover assets are the numerator and the covered bond liabilities are the denominator. Member States should be able to require a level of overcollateralisation to covered bonds issued by credit institutions located in the Member State concerned that is higher than the coverage requirement laid down in this Directive.