I've liked the McGarrigle sisters since the mid 70s when I first heard them and have always marvelled at how incredibly distinctive they are when they harmonise on other people's songs - eg I was listening to Linda Thompson's version of 'I Live not where I love' and there they are in the last bit of it and you INSTANTLY know it is them.

So I wondered, is it just the voices that are so distinctive or is it that they harmonise in a specific way that is different and makes them so recognisable? If it is the latter, what is it that they do that is different?