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Same Day Rule: there's a special rule for deciding which shares you've sold when you buy more shares of the same class in the same company on the same day. This is often called the same day rule. It prevents you making a gain or loss on shares you've sold and bought on the same day.

Securities: a security is an asset that, broadly speaking, has a financial value that is negotiable and is interchangeable with other similar assets. They're usually assets such as stocks and bonds. You're given a certificate - or there's electronic evidence - that shows you're the owner.