Type of recessed barrier
A ha-ha ( or ), also known as a sunk fence, blind fence, ditch and fence, deer wall, or foss, is a recessed landscape design element that creates a vertical barrier (particularly on one side) while preserving an uninterrupted view of the landscape beyond from the other side. The name comes from viewers' surprise when seeing the construction.
The design can include a turfed incline that slopes downward to a sharply vertical face (typically a masonry retaining wall). Ha-has are used in landscape design to prevent access to a garden by, for example, grazing livestock, without obstructing views. In security design, the element is used to deter vehicular access to a site while minimising visual obstruction.
Etymology.
The name ha-ha is of French origin, and was first used in print in Dezallier d'Argenville's 1709 book "The Theory and Practice of Gardening", in which he explains that the name derives from the exclamation of surprise that viewers would make on recognising the optical illusion. 
 ;templatestyles src="Template:Blockquote/styles.css" / ;Grills of iron are very necessary ornaments in the lines of walks, to extend the view, and to show the country to advantage. At present we frequently make thoroughviews, called "Ah, Ah", which are openings in the walls, without grills, to the very level of the walks, with a large and deep ditch at the foot of them, lined on both sides to sustain the earth, and prevent the getting over; which surprises the eye upon coming near it, and makes one laugh, "Ha! Ha!" from where it takes its name. This sort of opening is, on some occasions, to be preferred, for that it does not at all interrupt the prospect, as the bars of a grill do.
The name "ha-ha" is attested in toponyms in New France from 1686 (as seen today in Saint-Louis-du-Ha! Ha!), and is a feature of the gardens of the Château de Meudon, circa 1700.
In a letter to Daniel Dering in 1724, John Perceval (grandfather to the prime minister Spencer Perceval), observed of Stowe:
 ;templatestyles src="Template:Blockquote/styles.css" / ;What adds to the beauty of this garden is, that it is not bounded by walls, but by a ha-hah, which leaves you the sight of the beautiful woody country, and makes you ignorant how far the high planted walks extend.
In the 18th century, they were often called a sunken or sunk fence, at least in formal writing, as by Horace Walpole, George Mason, and Humphry Repton. Walpole also referred to them as Kent-fences, named after William Kent.
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! Has!" to express their surprise at finding a sudden and unperceived check to their walk."
Thomas Jefferson, describing the garden at Stowe after his visit in April 1786, also uses the term with exclamation marks: "The inclosure is entirely by "ha! ha!""
George Washington called it both a "ha haw" and a "deer wall".
Origins.
Before mechanical lawn mowers, a common way to keep large areas of grassland trimmed was to allow livestock, usually sheep, to graze the grass. A ha-ha prevented grazing animals on large estates from gaining access to the lawn and gardens adjoining the house, giving a continuous vista to create the illusion that the garden and landscape were one and undivided.
The basic design of sunken ditches is of ancient origin, being a feature of deer parks first found in Anglo-Saxon England. The deer-leap or consisted of a ditch with one steep side surmounted by a pale (picket-style fence made of wooden stakes) or hedge, which allowed deer to enter the park but not to leave. Since the time of the Norman conquest of England the right to construct a deer-leap was granted by the king, with reservations made as to the depth of the foss or ditch and the height of the pale or hedge. On Dartmoor, the deer-leap was known as a "leapyeat".
In Britain, the ha-ha is a feature of the landscape gardens laid out by Charles Bridgeman and William Kent and was an essential component of the "swept" views of Capability Brown. Horace Walpole credits Bridgeman with the invention of the ha-ha but was unaware of the earlier French origins.
 ;templatestyles src="Template:Blockquote/styles.css" / ;The contiguous ground of the park without the sunk fence was to be harmonized with the lawn within; and the garden in its turn was to be set free from its prim regularity, that it might assort with the wilder country without.
During his excavations at Iona in the period 1964–1984, Richard Reece discovered an 18th-century ha-ha designed to protect the abbey from cattle. Ice houses were sometimes built into ha-ha walls because they provide a subtle entrance that makes the ice house a less intrusive structure, and the ground provides additional insulation.
Examples.
Most typically, ha-has are still found in the grounds of grand country houses and estates. They keep cattle and sheep out of the formal gardens, without the need for obtrusive fencing. They vary in depth from about (Horton House) to (Petworth House).
Beningbrough Hall in Yorkshire is separated from its extensive grounds by a ha-ha to prevent sheep and cattle from entering the Hall's gardens or the Hall itself.
An unusually long example is the ha-ha that separates the Royal Artillery Barracks Field from Woolwich Common in southeast London. This deep ha-ha was installed around 1774 to prevent sheep and cattle, grazing at a stopover on Woolwich Common on their journey to the London meat markets, from wandering onto the Royal Artillery gunnery range. A rare feature of this east-west ha-ha is that the normally hidden brick wall emerges above ground for its final 75 yards (70 metres) or so as the land falls away to the west, revealing a fine batter to the brickwork face of the wall, thus exposed. This final west section of the ha-ha forms the boundary of the Gatehouse by James Wyatt RA. The Royal Artillery ha-ha is maintained in a good state of preservation by the Ministry of Defence. It is a Listed Building, and is accompanied by Ha-Ha Road that runs alongside its full length. There is a shorter ha-ha in the grounds of the nearby Jacobean Charlton House. The Royal Crescent row of 30 terraced houses in Bath, Somerset, which were built between 1767 and 1774 in the Georgian architecture style, also feature a large ha-ha that provides an uninterrupted view of Royal Victoria Park.
In Australia, ha-has were also used at Victorian-era lunatic asylums such as Yarra Bend Asylum, Beechworth Asylum, and Kew Lunatic Asylum in Victoria, and the Parkside Lunatic Asylum in South Australia. From the inside, the walls presented a tall face to patients, preventing them from escaping, while from outside they looked low so as not to suggest imprisonment. For the patients themselves, standing before the trench, it also enabled them to see the wider landscape. Kew Asylum has been redeveloped as apartments; however some of the ha-has remain, albeit partially filled in.
Ha-has were also used in North America. Only two historic installations remain in Canada, one of which is on the grounds of Nova Scotia's Uniacke House (1813), a rural estate built by Richard John Uniacke, an Irish-born Attorney-General of Nova Scotia.
Mount Vernon, the plantation of George Washington, incorporates ha-haws on its grounds as part of the landscaping for the mansion built by George Washington’s father, Augustine Washington. A later American president, Thomas Jefferson, "built a ha-ha at the southern end of the South Lawn [of the White House], which was an eight-foot wall with a sunken ditch meant to keep the livestock from grazing in his garden."
A 21st-century use of a ha-ha is at the Washington Monument to minimise the visual impact of security measures. After 9/11 and another unrelated terror threat at the monument, authorities had put up jersey barriers to prevent large motor vehicles from approaching the monument. The temporary barriers were later replaced with a new ha-ha, a low 0.76 m (30-inch) granite stone wall that incorporated lighting and doubled as a seating bench. It received the 2005 Park/Landscape Award of Merit.
Legal.
Personal injury.
Due to the hidden nature of ha-has, they can pose potential injury to the public (especially considering their initial designs were to be invisible).
References.
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 Media related to at Wikimedia Commons