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The no symbol (also known as a prohibition sign, no sign, circle-backslash symbol, nay, interdictory circle, or universal no) is a circle with a diagonal line through it (running from top left to bottom right), surrounding a pictogram used to indicate something is not permitted. The no symbol is generally colored red, but is sometimes another color, typically black.
The Unicode code point for the prohibition sign (also called the no symbol) is
U+20E0;, Combining Enclosing Circle Backslash ( ⃠ ). It is a combining character, which means that it appears on top of the character before it, so putting d⃠ will show d⃠. (Note: This will appear only if the character exists in an available font.) It also appears in the Webdings and Wingdings 2 fonts.
According to the ISO standard (and also under a UK Statutory Instrument), the red area must take up at least 35% of the total area of the sign within the outer circumference of the "prohibition sign". So 35% of everything within the outer edge of the "no symbol" must be the symbol itself. Additionally, for printed signs under the UK rules, the width of a "no symbol" is set at 80% the height of the area it is printed to.
The "prohibition" symbol is used on traffic signs, so that drivers can interpret traffic laws quickly while driving. For example:
By analogy, the sign is used in public places to refer to prohibited actions not having to do with traffic:
It is also used on packages sent through the mail, and sealed boxes of merchandise that are sold in stores. Using a graphical symbol is useful when the item must be handled by people who may not all understand the same language. For example:
In product documentation, this may be accompanied by drawings of the product being threatened by the prohibited items: for instance, a cartoon of a floppy disk being menaced by horseshoe magnets.
It is also used on clothing, linens, and other household products to indicate the care, treatment or cleaning of the item. For example:
Also, many companies use the "prohibition sign" when describing the services they offer, e.g. an insect deterrent spray brand symbol showing the "prohibition sign" over a mosquito. The Ghostbusters logo is a fictional example of this, although it uses a mirror image of the no symbol instead of the ISO 3864-1 version.
While the "prohibition sign" has been so widely used in advertising and promotions that now many variations of the design (for example, flipped left to right or varied in hue) are recognized by the public and considered acceptable, it is still governed by local and international standards. The standard definition of the no symbol comes from the International Organization for Standardization.
In 2002, ISO 3864-1 was published (a revision of a standard first published in 1984). The introduction includes language on the need for using as few words as possible to convey information.
ISO 3864-1 sets the rules for the color and shape of safety signage, as well regulating the incorporation of text according to viewing distance and sign size. The range of color and shape defined in this standard for the "prohibition symbol" or "no symbol" is defined as "a prohibition surround shape (red circular band with a red slash going from the upper left to the lower right) over the top of a black graphical symbol."
Since the standard is not published freely (ANSI sells it for US$104, while ISO sells it for CHF 88.00) and many examples seen in public diverge from the standard, many symbols and public signs using it have not actually been based on the official standard, but instead on inexact copies or interpretations of signs or symbols seen by each graphic artist. This has led to wide variations in color and dimensions. The most common variations seen are brighter reds than specified in the standard and either a much thinner width for the line and slash, or about the right dimensions but using the same width for the slash as the circle (the standard specifies a width that is 80% as wide as the circle). For example, compare Image:No smoking symbol.png (a non-compliant representation) with Image:ProhibitionSign.svg (a representation drawn using the ISO standard).
In contrast, a blue circle with no line through it, is used as a Mandatory Action symbol, indicating that the activity represented inside the circle is mandatory, and must be executed. 
The "no" symbol does not always have the "slash" across it; in all traffic uses other than no turn, "no" signs placed on roads are illustrated just with the prohibited action circled in red. An exception to this is the "No stopping" or "clearway" variant, which is illustrated with a red circle with a blue background and two diagonal "slashes" forming an X. However, according to the UK highway code, signs with red circles are mostly prohibitive, and classifies these signs as "signs giving orders" rather than prohibition signs.