Probably the main feature of property interests is that, to use an economist's expression, they are heterogeneous, or, in plain English, they are all different. Apart from joint interests in the same property, there will probably never be two interests which are exactly the same, and even though there may be row upon row of semi-detached houses which are apparently similar, they will all be on different sites. Although this may seem a minute distinction between them, it nevertheless means that each of the properties will be different.