"The vestment proper to the priest celebrant at Mass and other sacred actions directly connected with Mass is, unless otherwise indicated, the chasuble, worn over the alb and stole" (General Instruction of the Roman Missal, 337). Like the stole, it is normally of the liturgical colour of the Mass being celebrated.
The chasuble originated as a sort of conical poncho, called in Latin a casula or "little house", that was the common outer traveling garment in the late Roman Empire. It was simply a roughly oval piece of cloth, with a round hole in the middle through which to pass the head, that fell below the knees on all sides. It had to be gathered up on the arms to allow the arms to be used freely.
In its liturgical use in the West, this garment was folded up from the sides to leave the hands free. Strings were sometimes used to assist in this task, and the deacon could help the priest in folding up the sides of the vestment. Beginning in the 13th century, there was a tendency to shorten the sides a little. In the course of the 15th and the following century, the chasuble took something like its modern form, in which the sides of the vestment no longer reach to the ankle but only, at most, to the wrist, making folding unnecessary.
At the end of the sixteenth century the chasuble, though still quite ample and covering part of the arms, had become less similar to its traditional shape than to that which prevailed in the nineteenth and early twentieth centuries, when the chasuble was reduced to a broad scapular, leaving the whole of the arms quite free, and was shortened also in front and back. Additionally, to make it easier for the priest to join his hands when wearing a chasuble of stiff (lined and heavily embroidered) material, in these later centuries the front was often cut away further, giving it the distinctive shape often called fiddleback. Complex decoration schemes were often used on chasubles of scapular form, especially the back, incorporating the image of the Christian cross or of a saint; and rich materials such as silk, cloth of gold or brocade were employed, especially in chasubles reserved for major celebrations.
In the 20th century, there began to be a return to an earlier, more ample, form of the chasuble, sometimes called "Gothic", as distinguished from the "Roman" scapular form. This aroused some opposition, as a result of which the Sacred Congregation of Rites issued on 9 December 1925 a decree against it,De forma paramentorum which it explicitly revoked with the declaration Circa dubium de forma paramentorum of 20 August 1957, leaving the matter to the prudent judgement of local Ordinaries. There exists a photograph of Pope Pius XI wearing the more ample chasuble while celebrating Mass in Saint Peter's Basilica as early as 19 March 1930.