The ha-ha
A sunk fence: a ditch with a retaining wall on the house side, its top level with the lawn. From the house the lawn seems to run on into the park, but cattle cannot cross. Shown in section and in a short length.
The rules it illustrates
The ha-ha: a boundary that cannot be seen. In place of a wall, bound the garden with a ditch whose side towards the garden is a vertical retaining wall and whose other side slopes up to the field, so that cattle are kept out and the view from the house runs on uninterrupted into the park.
Where it appears in the book
In the styles English landscape garden.
In the chapters Gardens and Landscape.