Ruskin was in Stones of Venice seeking to trace the development of Gothic from Romanesque (a term originating in linguistics and defined by Ruskin as including Lombard and Byzantine styles) and its decline into renaissance classicising. The relationship between the Lombard School of the North (as seen in San Michele Pavia and St Ambrogio), the Arab School, and the Byzantine Style in the development of Gothic was an important question for Ruskin at this time. See for example Works, 9.34, Works, 9.39 and following, Works, 9.166 and following. Hence his interest in the stilted arch which was a significant in the early stages of the meeting of the three styles in Venice.
At Notebook M p.47L Ruskin distinguishes between Byzantine and Romanesque, and sees Pisa as making the transition from Romanesque to Gothic.
See also Pisa - Pistoya
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