Tuscan order A plain astragal or taenia ringed the column beneath its plain cap. Palladio agreed in essence with Serlio: "The Tuscan, being rough, is rarely used above ground except in one-storey buildings like villa barns or in huge structures like Amphitheatres and the like which, having many orders, can take this one in place of the Doric, under the Ionic." Unlike the other authors Palladio found Roman precedents, of which he named the arena of Verona and the Pula Arena, both of which, James Ackerman points out, are arcuated buildings that did not present columns and entablatures. A striking feature is
Ionic compound by a specific equilibrium distance. If the electronic structure of the two interacting bodies is affected by the presence of one another, covalent interactions (non-ionic) also contribute to the overall energy of the compound formed. Ionic compounds are rarely purely ionic, i.e. held together only by electrostatic forces. The bonds between even the most electronegative/electropositive pairs such as those in caesium fluoride exhibit a small degree of covalency. Conversely, covalent bonds between unlike atoms often exhibit some charge separation and can be considered to have a partial ionic character. The circumstances under which a compound will have ionic or covalent