In the fairy belief of Lowland Scotland and the Borders, the fae were divided into two hosts: the Seelie Court and the Unseelie Court. 'Seelie' — from an old word for 'blessed' or 'happy' — were the kindlier fairies, who might reward a kindness, help the poor and the diligent, and give fair warning before they took offence. They were seen moving between the worlds at twilight, and a household that left out milk or kept the hearth swept might find itself quietly favoured.
The story
Yet the Seelie Court's goodwill was never to be presumed upon. They repaid insult and meanness without mercy, and even their gifts could carry a hidden price, for the logic of Faerie was not human kindness but exchange and propriety. Against them stood the Unseelie Court, wholly malevolent — the unforgiven dead and harmful spirits who attacked travellers by night. The division preserved a careful folk-wisdom: the Good Neighbours might be benign, but only so long as they were respected and never named or crossed.