Spanish chestnuts (Castanea sativa), also called sweet chestnut or marron, are long-lived trees cultivated around the globe for their nuts.
Gretna Green is a small Scottish village just across the border from England.
As the laws regarding marriage differed in England and Scotland, underaged English couples who wished to marry without their parents' consent would traditionally elope to Gretna Green. Scottish law also permitted weddings to be conducted by almost anybody, so long as there were two witnesses. Consequently, many weddings were performed at the old blacksmith's.
Clapham is in South London.
Hackney-coaches were carriages for hire, like today's taxis.
There were many hackney-coaches on the streets and they all looked alike, making it difficult to trace the movements of any particular one.
Barnet is now a part of North London. Colonel Forster looks here as Mr Wickham and Lydia might have stopped in Barnet if they were on their way to Scotland. It is also en route to Longbourn.
Hatfield is a town in Hertfordshire, on the way from London to Longbourn.