After one has observed a sufficient number of windows to provide a basis for comparisons, it becomes easy to tell not only the epoch to which they belong, but also, in most instances, whether they are early or late in that epoch. In England one is assisted by an unusual amount of reliable information from two sources, viz., old records and heraldic indications from the coats of arms which are so often displayed. There is so little sixteenth century glass in this country as to give but small opportunity to observe the characteristic Renaissance custom of placing the dates on the picture itself, which was then common in France. Of earlier windows, however, English records and a knowledge of heraldry give us the dates of many more than are obtainable for their contemporaries in France. By way of example, the original contracts date the glass at Beauchamp Chapel, Warwick, 1447; at King's College, Cambridge, 1527; at York, in the nave, 1338, and in the choir, 1405, &c. A comparative and historical study of their heraldic blazons gives us a date for many of the windows at Bristol and at Wells, and of more still in private houses. The duty of the glazier was to adorn the window embrasures constructed for him by the architect, and thus assist in the decoration of the church. It is obvious that the size and shape of these apertures must necessarily have had considerable, if not controlling, effect upon the styles and methods of the glazier. A glance at the conformation of these openings often tells the sub-divisions in which its glass belongs. During Norman times the window arch was round and the opening wide. In the Early English style the arch at the top becomes pointed and the embrasures narrower. When the Decorated time arrives several narrow lights are grouped together, separated only by slender stone mullions, and culminating under the pointed arch at the top in a group of gracefully adjusted small apertures called tracery lights. The Perpendicular architect did little but straighten out the lines of his predecessors, especially in the traceries, so that they, as well as the mullions, should produce the effect of upright parallels which gave this type its name. In the sixteenth century the Renaissance architect provided large windows, and the glazier filled them with great pictures of splendid colour.