An instrument of sasine (pronounced 'say-zin') is a legal document that records the transfer of ownership (usually a sale or an inheritance) of a piece of land or of a building. It will normally detail the names of the new and previous owners and will give a basic description of the property transferred. There will usually be an indication of the price paid for the property. Sasines can also give you information about family history, particularly where an individual is passing land to another family member, or where the family designation is revealed (for example 'John Campbell of X'). Sometimes information given in one sasine will give you clues as to earlier titles in the chain and so lead you back to the earlier history of the ownership of a building or piece of land.
How many people had sasines?
Very few. This record is essentially about the 'haves' of Scottish society rather than the 'have nots'. Until the 20th century, when owner-occupation became more common, only a tiny proportion of Scots owned the house they lived in or the land that they farmed. Most rented their property and will not appear in registers of sasines.