Q: What is a scapular?
A: A scapular is part of a monk's or nun's habit: a strip of fabric protecting back and front of their habit. A hole is inserted in the middle so that it could hang over the shoulders. It got its name precisely from the Latin shoulder, meaning 'scapula'. It came to symbolize the gentle yoke of Christ. Third Order members (fifteenth century) were required to wear the scapular. Since they were lay people living in the world and did not wear a habit, the scapular was reduced in size, and eventually miniaturized (sixteenth century). It is made up of two small double squares of cloth (sometimes only one) of two-three inches square, suspended from the shoulders by two strings. It is usually the same color as the habit of the congregation the Third Order members (lay people affiliated to religious orders) belong to. The scapular has the image of Our Lady or a symbol or aspect of the particular devotion it recalls. Scapulars are sacramentals giving the wearer God's protection through the intercession--in the name and merits of Christ--of the church and its Saints. Presently there are approximately twenty scapulars, among them eight Marian. Here is the list of the Marian scapulars. Most of them have their origin in a vision.
Please have a look at Your Questions: Devotions--Scapular where you will find three features: one on the blue, one of the black, and one on the brown scapular (Mount Carmel).
The image on this page is a woodcut, Our Lady of Carmel, from a Spanish pamphlet published in 1758 by Jaime Esteva under the title "Jesus, Maria, Joseph: El mistico tri-estrellado Monte del Carmelo." Imprenta de San Juan de Bezares, Barcelona. Artist unknown.