Some jurisdictions allow contract law to overrule the status of legal tender, allowing (for example) merchants to specify that they will not accept cash payments. Coins and banknotes are usually defined as legal tender in many countries, but personal cheques, credit cards, and similar non-cash methods of payment are usually not. Some jurisdictions may include a specific foreign currency as legal tender, at times as its exclusive legal tender or concurrently with its domestic currency. Some jurisdictions may forbid or restrict payment made by other than legal tender. In some jurisdictions legal tender can be refused as payment if no debt exists prior to the time of payment (where the obligation to pay may arise at the same time as the offer of payment). For example, vending machines and transport staff do not have to accept the largest denomination of banknote. Shopkeepers may reject large banknotes, which is covered by the legal concept known as invitation to treat.