For centuries, the Daughters of Charity were known by their distinctive white cornette. However, St. Louise de Marillac, the Community’s co-founder, did not wear it. The cornette did exist in Louise’s time. References to it, and how it should be worn, can be found throughout the letters of both St. Louise and St. Vincent de Paul. Two footnotes in volume 2 of Vincent de Paul, Correspondence, Conferences, and Documents, (CCD), edited and translated by Sister Marie Poole, D.C. and others, give the answer to why Louise never wore the cornette.
Letter 530 – to St. Louise
v.2 p.198-199, footnote #1
“Sister Barbe Bailly, secretary to Saint Louise in the 1650s. stated in her notes that Saint Louise put on the habit of the Daughters of Charity one Pentecost Sunday and became so ill from doing so that she bad to return to her previous headdress. Although 1639 has been mentioned as the year this took place, we believe 1641 is more reasonable …”
Letter 534 – to St. Louise
Tuesday morning 
CCD v.2 p.206, footnote #3
“The costume of the first Daughters of Charity. almost all natives of the environs of Paris, was the one they were wearing when they presented themselves to Saint Louise to become members of the Little Company. Those who came from farther away used to dress, for the sake of uniformity, like the village women of the area surrounding Paris. Their habit was similar to that worn by the Sisters until 1964; however, the dress was gray, the collar shorter and only a toquois or toquet (small brimless hat) covered the bead. In the mind of the Holy Founder, the Daughters of Charity were, and were to remain, village girls. He wished them to be laywomen and not religious and. consequently, intended that they be dressed as “ordinary women,” according to his own expression. However, since the toquois gave poor protection from the weather, in 1646 the Saint allowed the more delicate among the Sisters, and in particular Sister Jeanne Lepeintre, who suffered from eye trouble, to add to their headpiece, as did many village women, the white cornette, an unstarched piece of material raised up in front and falling on both sides. The use of the cornette became generalized, and in 1685 Edme Jolly, the third Superior General, made it obligatory in order to remedy what might be shocking in a Community: a disparity of headdress. During the second half of the eighteenth century, the cornette became larger and, in the nineteenth century, starching was allowed to give it more consistency. Saint Louise did not dress like her Daughters. With Saint Vincent’s permission, she wore the usual costume of devout widows.”