Historically, the difference has been explained by whether the person taking it believes in 'divine mercy and vengeance', as an oath is believed to 'invoke divine vengeance' if the person does not speak the truth.
An affirmation is a solemn declaration of the truth without reference to anything divine, so it may be appropriate if the person is an atheist or agnostic (and where statutory provision permits its use).
An affirmation can also be used if it is not practical or convenient to accommodate the formalities required by a particular religious faith.
For further information please see the Law Society’s publication Execution of Documents (Third Edition), available to purchase from the Law Society’s online bookshop.
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Disclaimer: While every effort has been made to ensure the accuracy of the information in this article, it does not constitute legal advice and cannot be relied upon as such. The Law Society does not accept any responsibility for liabilities arising as a result of reliance upon the information given.