The scapular, from the Latin, scapula; in English, shoulder, forms a part, and now the most important part, of the habit of the monastic orders. Other orders and numerous religious congregations, both male and female, have also adopted the scapular from the monastic orders. It is usually worn over the habit or soutane.

The Scapular consists essentially of a piece of cloth about the width of the breast from one shoulder to the other, i.e., about fourteen to eighteen inches, and of such a length that it reaches not quite to the feet in front and behind. There are also shorter forms of the scapular. In the middle is the opening for the head, the scapular thus hanging down from two narrow connecting segments resting on the shoulders. Originally the longitudinal segments of cloth were connected by cross segments passing under the arms - a form which exists even today. In former times also two segments of cloth hung over the shoulders, which they covered, and thus formed a cross with the longitudinal segments over the breast and back.

The Small Scapulars

The small scapulars consist essentially of two quadrilateral segments of woolen cloth, about two and three-quarter inches long by two inches wide, connected with each other by two strings or bands in such a manner that, when the bands rest on the shoulders, the front segment rests before the breast, while the other hangs down an equal distance at the back.

The two segments of cloth need not necessarily be equally large, various scapulars having the segment before the breast of the above dimensions while the segment at the back is much smaller.

The material of these two essential parts of the scapular must be of woven wool; the strings or bands may be of any material, and of any one colour. The colour of the segments of woollen cloth depends on the colour of the monastic habit, which it to a certain extent represents, or on the mystery in honour of which it is worn.

On either or both of the woollen segments may be sewn or embroidered becoming representations or other decorations (emblems, names etc.) of a different material.

Several scapulars may be attached to the same pair of strings or bands; each scapular must of course be complete, and must be attached to both bands.

Only at the original reception of any scapular is either the blessing or the investment with such by an authorized priest necessary. When a person needs a new scapular, he can put on an unblessed one. If the investment with a scapular be inseparably connected with reception into a confraternity, the reception and enrollment must take place on the same occasion as the blessing and investment.

To share in the indulgences and privileges of a scapular, one must wear it constantly; it may be worn over or under one's clothing and may be laid aside for a short time, if necessary.

Should one have ceased wearing the scapular for a long period, even through indifference, one gains none of the indulgences during this time, but, by simply resuming the scapular, one again participates in the indulgences, privileges, etc.

Every scapular, which is not merely an object of private devotion, for there are also such, but is also provided with an indulgence, must be approved by the ecclesiastical authorities, and the formula of blessing must be sanctioned by the Congregation of Rites.

Text Source:
The Catholic Encyclopedia
Scapular