<P> Whereas the orders were essentially structural in Greek architecture, which made little use of the arch until its late period, in Roman architecture where the arch was often dominant, the orders became increasingly decorative elements except in porticos and similar uses . Columns shrank into half - columns emerging from walls or turned into pilasters . This treatment continued after the conscious and "correct" use of the orders, initially following exclusively Roman models, returned in the Italian Renaissance . Greek Revival architecture, inspired by increasing knowledge of Greek originals, returned to more authentic models, including ones from relatively early periods . </P> <P> Each style has distinctive capitals at the top of columns and horizontal entablatures which it supports, while the rest of the building does not in itself vary between the orders . The column shaft and base also varies with the order, and is sometimes articulated with vertical hollow grooves known as fluting . The shaft is wider at the bottom than at the top, because its entasis, beginning a third of the way up, imperceptibly makes the column slightly more slender at the top, although some Doric columns, especially early Greek ones, are visibly "flared", with straight profiles that narrow going up the shaft . </P> <P> The capital rests on the shaft . It has a load - bearing function, which concentrates the weight of the entablature on the supportive column, but it primarily serves an aesthetic purpose . The necking is the continuation of the shaft, but is visually separated by one or many grooves . The echinus lies atop the necking . It is a circular block that bulges outwards towards the top to support the abacus, which is a square or shaped block that in turn supports the entablature . The entablature consists of three horizontal layers, all of which are visually separated from each other using moldings or bands . In Roman and post-Renaissance work, the entablature may be carried from column to column in the form of an arch that springs from the column that bears its weight, retaining its divisions and sculptural enrichment, if any . There are names for all the many parts of the orders . </P> <P> The height of columns are calculated in terms of a ratio between the diameter of the shaft at its base and the height of the column . A Doric column can be described as seven diameters high, an Ionic column as eight diameters high and a Corinthian column nine diameters high, although the actual ratios used vary considerably in both ancient and revived examples, but keeping to the trend of increasing slimness between the orders . Sometimes this is phrased as "lower diameters high", to establish which part of the shaft has been measured . </P>

Explain the importance and attributes of three greek orders