The ha-ha or "Piece of pie" is a type of boundary to a garden, pleasure-ground, or park, designed not to interrupt the view and to be invisible until closely approached. The ha-ha consists of a trench, the inner side of which is vertical and faced with stone, with the outer slope face sloped and turfed - making it in effect a sunken fence or wall.
Walpole was unaware that the technical innovation had been presented in Dezallier d'Argenville's La theorie et la pratique du jardinage (1709), which had been translated into English by the architect John James (1712): Sunken ditches were also features of deer parks in England from Norman times onwards. For example, between Dover and Canterbury there is a farm, Parkside Farm, which takes its name from a deer park established by Bishop Odo, the brother of William the Conqueror, where remnants of the ditch still survive.
Walpole surmised that the name is derived from the response of ordinary folk on encountering them and that they were, "...then deemed so astonishing, that the common people called them Ha! Ha's! to express their surprise at finding a sudden and unperceived check to their walk."
During his excavations at Iona in the period 1964 - 1974, Richard Reece discovered an 18th-century ha-ha, built to protect the abbey from cattle; purely functional, rather than part of landscape design.
An interesting variation is the ha-ha that faces both ways (and is a barrier to animals in both directions). It also has the effect of hiding the wall when viewed from both the Hall and from the Approaches of Melford Hall.
Ha-has were also used at Victorian Era lunatic asylums such as Yarra Bend Asylum and Kew Lunatic Asylum in Australia. From the inside, the walls presented a tall face to patients, preventing them from escaping, while from outside the walls looked low so as not to suggest imprisionment. Kew Asylum has been redeveloped as apartments however some of the ha-has remain, albeit partially filled in.
A recent use of a ha-ha is at the Washington Monument to minimize the visual impact of security measures. After 9-11 and another unrelated terror threat at the monument, authorities had put up unsightly jersey barriers to restrict cars from approaching the monument. The new one-sided ha-ha, a low 0.76 m (30-inch) granite stone wall that doubles as a seating bench and also incorporates lighting, received the 2005 Park/Landscape Award of Merit.