The change which is represented in Gothic is due to several causes: (a) development of vaulting ribs; (b) the general use of the pointed arch; (c) reapplication of the Roman principle of concentration of vaulting strains upon four points; (d) the development of a buttress system to reinforce the main parts of the strain, and (e) the development of window openings both as to their size and ornamentation. =Periods of Gothic.=--The period of Gothic covers the thirteenth, fourteenth, and fifteenth centuries. The variations which it presented in these several centuries are often characterised by the changes in the treatment of the windows. Thus, in France, they have been divided [Illustration: SCULPTURED DETAIL FROM DOORWAY OF AMIENS CATHEDRAL. P. 269] [Illustration: SKELETON STRUCTURE SHOWING THE METHOD OF VAULTING, BY MEANS OF THE POINTED ARCH, AND THE CONCENTRATION OF THRUSTS AND COUNTER THRUSTS. P. 273] [Illustration: GOTHIC DETAIL. EARLY ENGLISH HASELEY, OXFORDSHIRE PERPENDICULAR CHRIST CHURCH CLOISTERS, OXFORD DECORATED SANDHURST, KENT EARLY HEREFORD CATHEDRAL EARLY BYLAND ABBEY, YORKSHIRE DECORATED HAMPTON POYLE, OXFORDSHIRE DECORATED CHAPTER HOUSE, SOUTHWELL PERPENDICULAR EWELME, OXON DECORATED YORK MINSTER] [Illustration: GOTHIC DETAIL TOOTH ORNAMENT CANTERBURY CATHEDRAL A CUSP PERPENDICULAR PERIOD DIAPER OF FOUR-LEAFED FLOWER A CUSP DECORATED PERIOD EARLY ENGLISH WESTMINSTER ABBEY A CUSP EARLY ENGLISH NICHE OF DECORATED PERIOD FINIAL KING’S COLLEGE, CAMBRIDGE ORIEL WINDOW] [Illustration: GOTHIC DETAIL FLAMBOYANT. DOORWAY HARFLEUR, NORMANDY BAY WINDOW COMPTON WINYATE, WARWICKSHIRE TRANSITION TO PERPENDICULAR HEADCORN, KENT GATEWAY BRASENOSE COLLEGE, OXFORD] [Illustration: GOTHIC DETAIL DECORATED PRESTON, KENT EARLY DECORATED DORCHESTER, OXFORDSHIRE DECORATED CHRIST CHURCH, OXFORD FLAMBOYANT S. OUEN, ROUEN EARLY ENGLISH JESUS COLLEGE, CAMBRIDGE] [Illustration: GOTHIC DETAIL LATER PERPENDICULAR ST. MICHAEL’S, OXFORD EARLY DECORATED PLATE TRACERY LATER DECORATED PLATE TRACERY STONE, KENT EARLY PERPENDICULAR KING’S SUTTON, NORTHAMPTONSHIRE] [Illustration: GOTHIC DETAIL ST. MARY’S CHURCH, DEVIZES, ENGLAND A. KING POST B. QUEEN POST C. BRACE OR STRUTS D. TIE-BEAMS E. PRINCIPAL RAFTERS F. RIDGE PIECES G. PURLINS H. COLLAR J. COMMON RAFTERS K. POLE-PLATE L. WALL-PLATE HALL OF WEARE GIFFORD, DEVONSHIRE, ENGLAND] [Illustration: By Courtesy of the Brooklyn Museum of Arts INTERIOR AND EXTERIOR VIEWS OF LICHFIELD CATHEDRAL SHOWING THE NAVE WIDENING. THE PIERS ARE SET ON A STRAIGHT LINE, AND AT EACH END OF THE NAVE ARE PERPENDICULAR UP TO THE CLERESTORY. MEANWHILE THE PIERS IN BETWEEN LEAN OUTWARD WITH INCREASING INCLINATION TOWARD THE CENTER OF THE NAVE. P. 280]