Women wearing trousers in the Western world after 1900
Since the adoption of trousers in Western Europe in Late Antiquity, trousers have been largely worn by men and not by women until the early 20th century.
During World War I, women wore their husbands' (suitably altered) trousers while they took on jobs previously assigned to men, and increasingly wore trousers as leisurewear in the 1920s and 30s. And for a period in the 1970s, trousers became quite fashionable for women. In the United States, this may be due to the passage of Title IX of the Education Amendments of 1972, which ruled that dresses could not be required of girls. Dress codes changed in public schools across the United States.
In the early 20th century aviatrices and other working women often wore trousers. Actresses Marlene Dietrich and Katharine Hepburn were often photographed in trousers from the 1930s. During World War II, women working in industrial work in war service wore trousers, and in the post-war era trousers were still common casual wear for gardening, socialising, and other leisure pursuits.
In Britain during the Second World War, because of the rationing of clothing, many women took to wearing their husbands' civilian clothes to work while their husbands were away in the armed forces. This was partly because they were seen as work garments, and partly to allow women to keep their clothing allowance for other uses. As the men's clothes wore out, replacements were needed, so that by the summer of 1944 it was reported that sales of women's trousers were five times more than in the previous year.
In 1919, Luisa Capetillo challenged the mainstream society by becoming the first woman in Puerto Rico to wear trousers in public. Capetillo was sent to jail for what was then considered to be a "crime", but, the judge later dropped the charges against her.
There are a number of religions that prohibit women from revealing their legs, requiring all women and often young girls to wear only trousers. The reasons behind such beliefs, which vary within each religion and culture, are generally due to modesty laws, which vary in each culture. Religion is the most common reason for a woman not to wear a skirt.
In Jewish belief, the space between a woman's legs is considered to be a private area, and therefore, must be covered by a garment. However, in other cultures wearing men's clothing is forbidden biblically under the prohibition of Lo Silbash ("A woman shall not wear that which pertaineth unto a man", Deuteronomy 22:5). In some Jewish communities, such as the Jews from Arab countries and Modern Orthodox communities, it is reasoned that trousers provide an extra form of modesty.
On 13 November 866, Pope Nicholas I wrote to King Boris I of Bulgaria: "Whether you or your women wear or do not wear trousers neither impedes your salvation nor leads to any increase of your virtue"(sive vos, sive feminae vestrae, sive deponatis, sive induatis femoralia, nec saluti officit, nec ad virtutum vestrarum proficit incrementum - Patrologia Latina, CXIX, 1002). Some members of the Society of Saint Pius X have spoken of the preference of women's wearing skirts rather than trousers. Cardinal Siri's letter has also been cited as justification for women wearing skirts and dresses. In addition, Saint Thomas Aquinas, the Church's principal theologian, also taught that "outward apparel should be consistent with the estate of the person, according to the general custom. Hence it is in itself sinful for a woman to wear man's clothes, or vice versa; and it is expressly forbidden in the Law (Deuteronomy 22)..." 
Although many contemporary Mennonites have no dress code, among traditional, conservative Mennonites, sometimes referred to as "Old Order Mennonites", long skirts or dresses covering most of the legs are required. They also wear dresses and skirts because they believe men and women should be distinguished from one another. Conservative conferences usually demand that women wear a specific style of dress. This is usually in the style of the cape dress, with a double covering or "cape". Most non-conservative conferences allow for the wearing of trousers by women.
Many forms of dancing require females to wear skirts or dresses, either by convention or competition rules. In Scottish highland dancing for example, women don't wear trews, but instead either wear a skirt or dress including the Aboyne dress (for the national dances) or the kilt-based outfit for the Highland dances. However, tartan trews can be worn by women in the United States.
Most UK schools allow girl pupils to wear trousers, but many girls still wear skirts in primary and secondary schools, even where the choice of trousers is given. In the late 20th century, many schools began changing their uniform rules to allow trousers for girls amidst opposition to skirts only policies - the most publicised possibly being Jo Hale vs Whickam Comprehensive in 2000. Although commonly accepted that girls may wear trousers to school, no test case is known to have been brought before the courts, making the legal position uncertain on requiring skirts as part of girls' uniforms. The rule is still enforced in many schools, particularly independent and selective state schools.
In California, Government Code Section 12947.5 (part of the California Fair Employment and Housing Act) expressly protects the right to wear pants (American English for trousers)  Thus, the standard FEHA discrimination complaint form includes an option for "denied the right to wear pants." 
In 2013 a bylaw requiring women in Paris, France to ask permission from city authorities before dressing as “men,” including wearing trousers (with exceptions for those “holding a bicycle handlebar or the reins of a horse”) was declared officially revoked by France’s Women’s Rights Minister, Najat Vallaud-Belkacem. The bylaw was originally intended to prevent women from wearing the pantalons fashionable with Parisian rebels in the French Revolution. 
- L.W.N. Smith. Clothes Rationing in World War 2
- The Responses of Pope Nicholas I to the Questions of the Bulgars A.D. 866 (Letter 99), translated by W. L. North (1998) from the edition of Ernest Perels, in MGH Epistolae VI, Berlin, 1925, pp.568-600.
- Letter on trousers
- Men's Dress Worn By Women, Giuseppe Cardinal Siri
- Summa Theologica, II II, Q. 169, A.2
- Why do Apostolic Pentecostal women always wear dresses and have long hair?
- What Christian religions require certain types of clothing to be
- California Government Code Section 12947.5.
- Instructions to Obtain Right to Sue Notice by Mail, California Department of Fair Housing and Employment, February 2013.