The intervals formed by the regular spacing of the thick iron frame-bars are further emphasized by the placing of a coloured boss or small medallion midway between each. This arrangement in some form or other is almost universal in fourteenth century grisaille, the panels contained between the frame-bars being in fact the units of the design. Some of these bosses from Rouen are shown in Plates XXVI., XXVII., XXIX. Here they are purely fantastic in design, but elsewhere, as at York, they frequently have an heraldic motive or even take the form of shields of arms (Plate XVIII.). Heraldic motives are very commonly used too in the borders, as may be seen in the details from York Minster in Plate XVII., the charges from the shield being repeated all up the border, relieved against, or sometimes alternating with blocks of the colour of the field. Symbolic objects such as chalices are sometimes used in the same way, and occasionally we find borders formed of a succession of little figures under canopies, as in the very elaborate example from York in Plate XVIII. [Illustration: PLATE XXIV ST. LUKE, FROM CHOIR CLERESTORY OF ST. OUEN, ROUEN Fourteenth Century] [Sidenote: Quarries.] As the century proceeds quarries become much the commonest form of grisaille. In Plates XXV., XXVI. they are true quarries, but in the first quarter of the fourteenth century they are sometimes, as, for instance, at York, "bulged" round the central boss, thus forming a sort of cross between quarries and geometric glazing. Grisaille glazed in geometric patterns such as we find at Merton College, at Evreux, and in St. Pierre at Chartres belongs, I think, always to quite the early years of the period, and even then, as may be seen in Plate XXIII., it shows a decided leaning towards quarry-work, and indeed needs little but the straightening of the leads to convert it into quarries altogether. Continuous painted patterns are now the rule, as shown in Plate XXVI. The cross-hatched grounds disappear, and presently the silver stain is used (as here) to enrich the painting. It will be noticed that the trellis-like pattern produced by painting lines parallel to the leading is still retained. [Sidenote: The canopy.]