Medigap is insurance designed to fill some of the gaps in your Medicare coverage. Medigap policies are sold through private insurance companies. However, they are standardized and regulated by federal and state law. Under federal law, only ten standardized plans can be offered as Medigap plans. All ten must cover certain services, no matter in which state you live. Medigap policies pay most, if not all, Medicare coinsurance amounts. Some also provide coverage for deductibles and services that are not covered by Medicare. All ten plans are not available in every state from every carrier, and costs very from company to company.