柱式是指一整套古典建筑立面形式生成的原则。基本原理就是以柱径为一个单位，按照一定的比例原则，计算出包括柱础（Base）、柱身（shaft）和柱头（Capital）的整个柱子的尺寸，更进一步计算出包括基座（Stylobate）和山花（Pediment）的建筑各部分尺寸。

15世纪以来，建筑学研究者发现了包括古希腊的三个柱式和古罗马的五个柱式。最早的柱式来源于古希腊，之后为古罗马所使用和修改。每种类型都有各异的柱头形式，包括过梁(额枋)、雕带和檐口。

按柱式雕刻的顺序，由最粗壮原始的到最苗条奢华的柱式依次是塔司干柱式(古罗马)、多立克柱式(古希腊和古罗马)、爱奥尼克柱式(古希腊)、爱奥尼克柱式(古罗马)、科林斯柱式(古希腊和古罗马)以及混合柱式(古罗马)。这其中有三种柱式:多立克、爱奥尼克和科林斯是由古希腊人发明的。随后古罗马人增加了比多立克柱式简单的塔司干柱式，以及比科林斯柱式还繁杂的组合柱式。

"An Order in architecture is a certain assemblage of parts subject to uniform established proportions, regulated by the office that each part has to perform". The Architectural Orders are the ancient styles of classical architecture, each distinguished by its proportions and characteristic profiles and details, and most readily recognizable by the type of column employed. Three ancient orders of architecture—the Doric, Ionic, and Corinthian—originated in Greece. To these the Romans added, in practice if not in name, the Tuscan, which they made simpler than Doric, and the Composite, which was more ornamental than the Corinthian. The Architectural Order of a classical building is akin to the mode or key of classical music, the grammar or rhetoric of a written composition. It is established by certain modules like the intervals of music, and it raises certain expectations in an audience attuned to its language.