No symbol
The general prohibition sign, also known informally as the no symbol, 'do not' sign, circle-backslash symbol, nay, interdictory circle, prohibited symbol, don't do it symbol, or universal no, is a red circle with a 45-degree diagonal line inside the circle from upper-left to lower-right. It is overlaid on a pictogram to warn that an activity is not permitted, or has accompanying text to describe what is prohibited. Appearance. According to the ISO standard (and also under a UK Statutory Instrument), the red area must take up at least 35 percent of the total  area of the sign within the outer circumference of the "prohibition sign". Under the UK rules the width of a "no symbol" is 80 percent the height of the printed area. For computer display and printing, the symbol is supported in Unicode by combining elements rather than with individual code points (see below). Uses. Motor vehicle traffic signage. The "prohibition" symbol is used on traffic signs, so that drivers can interpret traffic laws quickly while driving. For example: Non-motor traffic. The symbol's use is not limited to informing drivers of motorized vehicles, and is commonly used for other  forms of traffic: General prohibitions and warnings. The symbol is used for non-traffic purposes to warn or prohibit certain activities: Packaging and products. 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 to convey important warnings regardless of language. For example: In product documentation, the symbol 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: Promotional and advertising. Some 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 symbol with the slash going from upper right down to lower left. International standards. The official prohibition sign design characteristics are governed by regional and international standards. The symbol's canonical definition comes from the International Organization for Standardization which  published ISO 3864-1 in 2002, a revision of a standard first published in 1984. The current version was published in 2011. ISO 3864-1 sets the rules for the color, shape, and dimensions of safety signage. The regulations include the incorporation of text and pictograms, with reference to materials used, sign size, and viewing conditions. The introduction includes language on the need for using as few words as possible to convey information. Design specifics. The 3864 standard defines the color and design for the prohibition symbol. The verbal definition reads "circle with diagonal bar" with a red safety color , a white contrast color, and black for the graphical symbol (i.e. the pictogram). Dimensions per 3864-1. The symbol is defined as a circle, with the circular band having a thickness of 10% of the outer diameter of the circle. The inner diagonal line has a thickness of 8% of the outer diameter of the circle (i.e. 80% of the circle's line width). The diagonal is centered in the circle and at a 45 degree angle going from upper left to lower right. It is recommended to have a white outside border that is 2.5% to 5% of the  outer diameter of the circle. The circle and line are red, the background is white, and the pictogram or descriptive text is black. Color red per 3864-1. The standard defines the range of CIE x,y chromacity coordinates for the color red to be used, relative to the CIE 1931 2° standard observer. They also list equivalent colors for various common color systems such as Munsell, defining red as Munsell 7,5R 4/14. Relative to CSS colors for the web and sRGB, and assuming a white background of codice_1, the red color should be no lighter than codice_2, and  no darker than codice_3, with codice_4 being a useful choice in terms of good contrast and color. Variations. Non-conforming designs. Despite the fact that the ISO standard is freely available, it is not uncommon for graphic artists to improvise on the particular color and dimensions. As a result there is a wide variation of the symbol in common use, for instance using a lower contrast red than specified in the standard or using the same width for the diagonal line as the circle (the standard specifies the diagonal width is 80% as wide as the circle). For example, compare  (a non-conforming example) with (drawn using the ISO standard). Alternate or regional design differences. Circles with red borders and no slanted or diagonal line are used under the Vienna Convention on Road Signs and Signals to indicate "No entry to vehicles with the following characteristics" (often defined on a plate beneath) such as height, width, mass, or speed. The European Vienna Convention prohibits a diagonal line in the symbol for any sign other than no turning signs. An alternative use for red bordered circles is as a Mandatory Action Symbol type B. In many jurisdictions, (such as Germany) 'no  entry' is indicated by a solid red disc with white horizontal bar. See the general article on Prohibitory traffic signs. A blue filled circle with an illustration or legend means that a lane is restricted to a particular class of users (such as buses, cyclists, pedestrians) as shown, and no other traffic may use it. In contrast, a blue filled circle without a diagonal line through it is used as a Mandatory Action Symbol, indicating that the activity represented inside the circle is mandatory and must be executed. Unicode and fonts. Unicode combining character. The Unicode code point for the  prohibition sign is . It is a combining character, which means that it appears on top of the character immediately before it. Example: Putting codice_5 will display the letter W inside the prohibition sign: Emoji & other unicode versions. There are also several prohibition sign emojis and related unicode characters: In fonts. The symbol appears in a number of different fonts, such as Arial Unicode MS, and in dingbat fonts such Webdings and Wingdings 2. These are not necessarily "combining characters." In the case of Webdings and Wingdings 2, the character encoding does not match the Unicode standard, so if these fonts  are not present on the user's system, the symbol may not render correctly. This is particularly an issue in webpages. If the page designer wishes to use Webdings for instance, it is important to provide the font resource via the CSS @import font-face command. Similar-appearing symbols. Many fonts provide glyphs that look very similar to the prohibited symbol. The math symbol in particular is difficult to distinguish from the prohibited symbol. is another, though both of these have limited font support. There is much greater font support for the following similar looking math symbols and glyphs: Regarding  the last two, many fonts provide these glyphs as perfect circles, which if rotated 45° essentially matches the look of the prohibited symbol. The Cyrillic "capital barred O" has greater font support than the Greek capital theta.