Before 'Windows' there must have been 'Doors'

Whilst researching a house in Blackheath in London I discovered some interesting notation in the basement. These must have been calculations for the coal bill, scrawled onto the nearest thing to hand- the back of the door. I suppose rather than using a piece of paper, which would easily be lost in the dingy cellar, the tally was always to hand and could be easily updated, although I’m more of a windows man myself.