A sans-serif typeface family, one of the world's most commonly seen by virtue of its being provided with every version of Microsoft Windows since Windows 3.0 or so, and quite likely to be the typeface in which you are reading this right now.

Arial was developed for Microsoft by the Monotype corporation in the 1980s as a substitute for the long-established Linotype Helvetica, the hitherto most widely used sans face, since using Helvetica would have involved paying licensing fees to Linotype (as Apple chose to do).

Although there were a large number of effectively identical imitations of Helvetica in circulation (one for every printer manufacturer's firmware fonts, at least), Monotype, being a fairly reputable typographical design company, chose to develop a distinctly different face, while keeping the font metrics identical to Helvetica so that the two could be interchanged without otherwise affecting page layout. Arial is broadly speaking a descendent of Monotype's own Grotesque series, somewhat expanded. Although superficially (i.e. to people who aren't design anoraks) its resemblance to Helvetica is almost total, the two can be distinguished fairly clearly (in large enough sizes, at any rate) in several ways:

  • a - the tail on the descender has a marked curve to the right in Arial, and much less of one in Helvetica
  • C - the faces at the ends of the arms are parallel in Helvetica but perpendicular to the curve in Arial
  • G - the face at the end of the upper arm is parallel to the horizontal bar in Helvetica, but perpendicular to the curve in Arial; in Helvetica the lower arm curves all the way up to the end of the horizontal bar and then has a added vertical tail, while in Arial the lower arm turns into a straight vertical line up to the horizontal bar; there is no added tail
  • R - the tail of the letter starts further to the right in Helvetica than in Arial, curving almost to a vertical and then ending with a flick to the right; in Arial the tail is almost straight
  • t - the top of the riser is square in Helvetica, angled up to the right in Arial
The face has roman and italic forms at regular, bold and black weights. A condensed variant, Arial Narrow is available in roman and italic forms at regular and bold weights, while an extended version, Arial Unicode MS extends to a 4000 glyph subset of the Unicode character set in a vast array of alphabets, which goes some way beyond the aspirations that the Haas type foundry had when it developed Helvetica half a century ago; it is only produced in a single roman form.1 A rounded variant, Arial Rounded MT Bold, has been produced in roman bold only. The roman and italic faces at regular and bold weights are included in the Windows Core Fonts set and thus installed with practically every copy of Windows; the others have been supplied with various versions of Microsoft Office and other software over the years and can in extremis be purchased from MS. Two supplementary four-version sets containing characters outside the Latin-1 range, Arial Special G1 and G2, and counterparts for Arial Narrow, were produced in the 1990s for MS applications like Encarta and MapPoint; these have presumably now been made redundant by extended versions of the normal files and the Unicode set.

1. These refer to separately designed sets of glyphs for different forms and weights; your word processor or other application will undoubtedly generate emboldened and slanted versions of any face you throw at it where a proper set is not available.

Thanks to yesno for clarification re the licensing issues, and The Librarian for reminding me about Arial Unicode MS. Some information gleaned from the horse's mouth.