You have in these two series the primal representations of shaft arrangement in the Southern and Northern schools; while the group _b_, of which _b2_ is the double, set evenly, and _c2_ the double, set obliquely, is common to both. The reader will be surprised to find how all the complex and varied forms of shaft arrangement will range themselves into one or other of these groups; and still more surprised to find the oblique or cross set system on the one hand, and the square set system on the other, severally distinctive of Southern and Northern work. The dome of St. Mark's, and the crossing of the nave and transepts of Beauvais, are both carried by square piers; but the piers of St. Mark's are set square to the walls of the church, and those of Beauvais obliquely to them: and this difference is even a more essential one than that between the smooth surface of the one and the reedy complication of the other. The two squares here in the margin (Fig. XV.) are exactly of the same size, but their expression is altogether different, and in that difference lies one of the most subtle distinctions between the Gothic and Greek spirit,--from the shaft, which bears the building, to the smallest decoration. The Greek square is by preference set evenly, the Gothic square obliquely; and that so constantly, that wherever we find the level or even square occurring as a prevailing form, either in plan or decoration, in early northern work, there we may at least suspect the presence of a southern or Greek influence; and, on the other hand, wherever the oblique square is prominent in the south, we may confidently look for farther evidence of the influence of the Gothic architects. The rule must not of course be pressed far when, in either school, there has been determined search for every possible variety of decorative figures; and accidental circumstances may reverse the usual system in special cases; but the evidence drawn from this character is collaterally of the highest value, and the tracing it out is a pursuit of singular interest. Thus, the Pisan Romanesque might in an instant be pronounced to have been formed under some measure of Lombardic influence, from the oblique squares set under its arches; and in it we have the spirit of northern Gothic affecting details of the southern;--obliquity of square, in magnificently shafted Romanesque. At Monza, on the other hand, the levelled square is the characteristic figure of the entire decoration of the façade of the Duomo, eminently giving it southern character; but the details are derived almost entirely from the northern Gothic. Here then we have southern spirit and northern detail. Of the cruciform outline of the load of the shaft, a still more positive test of northern work, we shall have more to say in the 28th Chapter; we must at present note certain farther changes in the form of the grouped shaft, which open the way to every branch of its endless combinations, southern or northern.