There is widespread confusion over the question of just what is - and what is not - a kilt and how this traditional Scottish garment differs from a skirt or a kilt-skirt. In fact, this confusion has at times erupted into controversy. Recently, for example, the European Union, for purposes of establishing trade regulations within the EU, classified the Scottish kilt under the same category as skirts (that is, as a form of women's wear), resulting in a protest from outraged kilt manufacturers!  In addition, many Scotsmen take offense at the kilt being referred to as a skirt, or vice versa (with certain types of skirts commonly - and confusingly - being referred to as "kilts").
In what follows, this article has reference to the kilt in its modern form, as typically seen at modern Highland games gatherings in Scotland and throughout the world. Historical kilts can and have differed in several particulars (some quite substantial) from the modern-day kilt.
More specifically, the organizations which sanction and grade the competitions in Highland dancing and bagpiping all have rules governing acceptable attire for the competitors. These rules specify that kilts are to be worn (except that in the National dances, the female competitors will be wearing the Aboyne dress). The word kilt as used in this article refers to the usage of that term in these official competitions and as enforced by their judges. 
A skirt is a tube- or cone-shaped garment which is fastened at the waist and hangs down encircling and covering all or part of the legs. Within this meaning, the skirt can assume a wide variety of styles, lengths, shapes, etc, and can be made out of a wide variety of materials. Note also that the definition put forth here is restricted to the construction (tailoring) and the usage (as a body covering) of the garment and makes no mention of societal or other expectations.
Within this meaning of the term, there is one type of skirt known as a wrap-around skirt. This is a skirt which consists of a single piece of fabric cut so that it is open along a line from waist to hem and which can be fastened about the body by wrapping it around the waist and then securing the opening in a variety of ways, for example, with a zipper, or with buttons, clasps, snaps, buckles, etc.
In one type of wrap-around skirt, called a kilt-skirt (or kilted skirt), the two ends overlap considerably to form what are called aprons. These aprons are positioned in the front while the remaining length of the fabric (around the sides and in the back) is pleated (almost always knife pleated). The kilt-skirt will also exhibit a plaid pattern so as to resemble, at least in overall visual appearance, the look and feel of an authentic Scottish kilt. In some cases, the plaid patterns of these garments will very closely resemble those of official Scottish kilts, sometimes differing only in the alteration of a single colored thread in the pattern.
Even barring the techniques of deconstructionism, there is no simple way, without resort to artificial or social conventions, in which the word kilt (or the garment it represents) can be defined so as to clearly differentiate it from a skirt. That is, there is no simple definition of the word kilt which would include all such garments recognized to be kilts and exclude all types of skirts. The converse is true as well. Of course, such social conventions (such as that the kilt is intended for wear by men and the skirt is intended for wear by women) can be extremely important to a great many people (as witness the protests over the previously mentioned EU trade classifications). Nevertheless, important as they may be to many people, they are still just social conventions .
What is true of the modern kilt is even more true if one were to take into account the full range of kilt styles as seen throughout history.
Bearing in mind the above definitions and considerations, we can now set forth some of the more evident characteristics and features of the modern-day kilt.
First, a kilt is a garment made of twill woven worsted wool which in its overall appearance and construction resembles that of a wrap-around skirt (it would not be unfair to say that it is a type of wrap-around skirt). Within this class, the kilt will have overlapping front aprons and will be knife pleated around the sides and back (though see the section on historical kilts below). The kilt will also exhibit a special type of plaid pattern.
The plaid patterns of modern kilts are arranged in a set of horizontal and vertical stripes (defined by the thread count). These stripes are never set at a slant angle. In addition, the plaid patterns are (with extremenly rare exceptions) symmetrical, meaning that if the kilt fabric were turned by 90 degrees, the pattern would then be unchanged. And finally, the plaid patterns of the modern kilt are all registered with some designated body (such as the Scottish Tartans Society) or are otherwise officially recognized by an official Scottish clan society. This latter factor is an example of an artificial convention as mentioned above.
The kilt will (again, with very rare exceptions) be pleated to the sett or stripe. When pleated to the stripe (also called military pleating, a style often used by pipe bands), each pleat will be folded to show the same stripe in the middle. Pleating to the sett results in a kilt in which the pattern of the sett is repeated across the back. Both types of pleating commonly result in very deep pleats, requiring usage of a large amount of fabric (typically 6 to 8 yards).
The earliest surviving kilts (or remnants) which have survived date from the late 18th century and are regimental kilts. These kilts were box pleated garments with the pleats extending all the way around the circumference of the garment. Nor did they have a wrap-around construction.
Apart from the traditional kilt itself, perhaps the most well-known example of a garment which is frequently identified as a kilt (at least in the United States) is the ubiquitous Catholic school uniform skirt for use by girls in Catholic parochial schools. First coming into prominence in the post-World War II era, this style of school uniform became so widespread by the 1960s that it virtually defined a type.
This garment, which is essentially a plaid, pleated skirt, differs from the traditional Scottish kilt in several respects. First, the patterns exhibited on the Catholic school uniform skirts are, for the most part, not symmetrical. That is, although they are arranged horizontally and vertically (not set at an angle), the vertical stripes differ, sometimes significantly, from the horizontal striping. Still, some of the patterns in common use are, in fact, symmetrical or very nearly so.
In addition, although the patterns are typically given the names of Scottish clans (one pattern is called a MacDonald plaid, another a MacBeth plaid, for example), these names have nothing to do with the Scottish clans of those names nor are the patterns or colors anything like the "official" clan patterns recognized by the Scottish clans.
In addition to the differences in the patterns, the Catholic school uniform skirt differs from the Scottish kilt in its overall construction or appearance. The school uniform skirt is not a wrap-around garment with the characteristic overlapping front aprons of the kilt (or the kilt-skirt, for that matter), but instead the pleats extend the all the way around the skirt. Also, since this type of skirt is not pleated to the sett (if there can be said to be such), the pleats are not anywhere near as deep as those of the traditional Scottish kilt and the garment, as a result. will not use near as much material.
Finally, in the interests of having a garment which is economical and tough, these uniform skirts are made out of polyester (at least, as manufactured by the Dennis Uniform Company, the main supplier of such outfits in the United States).
In addition to the above-mentioned overall differences between the modern kilt and at least this one type of garment commonly (and mistakenly) referred to as a kilt, there are numerous differences in detail in the construction of the two types of garments. Just some of the more prominent of these differences in detail will be mentioned below.
A kilt is individually tailored to fit the body proportions of the wearer. From the waist to the widest portion of the seat, the pleats will be stitched down (though at least one manufacturer has been known to glue them down). From there, the pleats, which are pleated to the stripe or sett and not angled even slightly, will hang straight down and not flare out. The school uniform skirt does not have stitched down pleats and thus, since the seat is generally wider than the waist, it will tend to flare out.
Some other differences between a kilt and a kilt-skirt, are:
- unlike a skirt, the kilt is not hemmed, but is instead made on the selvedge;
- a kilt will have belt loops, whereas the kilt-skirt almost never does;
- on a kilt, the front aprons overlap left over right whereas on a kilt-skirt they overlap right over left;
- the kilt pin is not fastened through both layers of the kilt fabric, while the similar device on a kilt-skirt (also called a kilt pin) is usually fastened through both layers.
Other uses of the word 'kilt'
In recent years, the word 'kilt' has been applied, either directly or in some hyphenated or compound form, to quite a variety of unbifurcated skirt-like garments. In addition to the kilt-skirt, we have the neokilt™, the utilikilt™, the x-kilt, and an assortment of other garments called just plain kilts but which differ markedly from the traditional Scottish kilt described above. It is difficult, to say the least, to arrive at a definition which would include all of these types of non-traditional kilts while at the same time excluding other catgories of garments which are clearly not kilts of any type and are not referred to as such.
In some cases, such as the Catholic school uniform skirt discussed above, the garment, which is often referred to as a kilt, has a morphology quite different from that of the traditional Scottish kilt and is designed for wear by females. Other school uniform manufacturers offer a style of uniform skirt for girl students which is referred to above as a kilt-skirt (though sometimes these kilt-skirts do not exhibit plaid patterns), but which is referred to in their catalogues simply as a kilt
But many of these non-traditional kilt-like garments are designed for, and marketed to, men. In still other cases, skirt-like garments designed as men's wear and which have very little in common with the traditional Scottish kilt are nevertheless referred to as kilts. Taken as a whole, the only common characteristics these garments seem to share with the Scottish kilt is that they are unbifurcated lower body coverings containing pleated fabric, belt loops and front aprons that overlap left over right, that are specifically designed and intended for wear by men.
- News item in The Guardian regarding EU classification of kilts as womenswear
- Rules of the British Columbia Pipers Association - in which "acceptable highland dress" for solo pipers and pipe bands is specified
- Interview with SOBHD dance judge Heather Richendrfer - in part, a discussion of SOBHD required dance attire