The chasuble began as the paenula, a conical travelling cloak of the Roman world with a hole for the head, worn by everybody. In its original form it fell to the wrists and had to be gathered up over the arms to free the hands, which is why medieval illustrations show servers lifting the celebrant's vestment at the elevation.
Over several centuries the sides were cut away to solve that inconvenience, and the cutting went a long way. By the Baroque period the Roman form had become two stiff boards of ornamented fabric hanging front and back over the shoulders, with no material over the arms at all, a shape known in English as the fiddleback for the outline of its front panel. It carried heavy embroidery well and it was practical, and it became the norm across the Catholic world.
The nineteenth century liturgical movement objected. Reformers, including Augustus Pugin in England, argued that the full conical or ample Gothic form was the authentic garment and that the fiddleback was a mutilation, and they revived the older cut. Rome was initially unenthusiastic and restricted the revival, then relaxed, and by the middle of the twentieth century the fuller shapes were general. The argument has not gone away: the fiddleback returned to use in traditionalist celebrations and is now a visible marker of liturgical allegiance rather than a neutral choice of tailoring.
The chasuble carries the liturgical colour, and this is where the colour system is most visible: white or gold for feasts and the seasons of Christmas and Easter, red for martyrs and Pentecost, green for ordinary time, violet for Advent and Lent, and rose on two Sundays. Black survives for requiems in some places.