The first question the monks had to decide was that of lighting. Their answer was a momentous one. They decided to get additional light by raising the walls of transept and presbytery, and inserting immense clerestory windows. The nave was internally 67½ feet high. To get a big clerestory, they raised the south transept and presbytery to the height of 86 feet. Similar tall clerestory windows were being built, but in the Curvilinear style, in the choirs of Lichfield and Wells. But as yet clerestory windows were, as a rule, rather small; and some big churches of the Curvilinear period, such as Grantham and Patrington, had no clerestory at all. For what was to be the glory of the closing days of English Gothic architecture—the Lantern type of church—we must give the credit to the masons of Gloucester. And also for the new type of window. The new big window, occupying the whole breadth and nearly the whole height of the end wall of the south transept, had to be strengthened by cross-bars or transoms; and the tracery of the head had to be strengthened by the substitution of vertical straight lines, as far as possible, for curves. Here, then, we have the genesis of the Perpendicular or Rectilinear window. Then there was the question of the vault. If we can believe Abbot Froucester, so early as 1337 the monks put over the south transept not only one of the earliest lierne vaults in the kingdom, but one so accurately worked that the junctions of the ribs did not require to be masked by bosses: they mortised the ribs together with as much precision as if they were dealing with joints in cabinet work. And over the choir and under the tower they put up the most amazingly complicated lierne vault that was ever constructed: this, too, not later than 1351.