Georgian architecture is characterized by its proportion and balance; simple mathematical ratios were used to determine the height of a window in relation to its width or the shape of a room as a double cube. Regularity, as with ashlar (uniformly cut) stonework, was strongly approved, imbuing symmetry and adherence to classical rules: the lack of symmetry, where Georgian additions were added to earlier structures remaining visible, was deeply felt as a flaw, at least before Nash began to introduce it in a variety of styles. Regularity of housefronts along a street was a desirable feature of Georgian town planning. Until the start of the Gothic Revival in the early 19th century, Georgian designs usually lay within the Classical orders of architecture and employed a decorative vocabulary derived from ancient Rome or Greece.