Roman 51 is a term used to describe the style of architecture that was popular during the Roman Empire. It is characterized by its use of arches, columns, and domes. The two-letter acronym LI stands for 'Lintel and Impost, ' which are two key elements of this style of architecture. A lintel is a horizontal beam that is placed over an opening, such as a doorway or window, while an impost is a block of stone that supports the arch above it. Together, these two elements form the basis of the Roman 51 style of architecture.