What is another word for under side?
Pronunciation: [ˌʌndə sˈa͡ɪd] (IPA) 
There are several synonyms for the word "under side," which refer to the underside or bottom of an object. These include the words underside, underside view, undersurface, bottom, underbelly, lower surface, and ventral surface. The term underside is commonly used to describe the bottom or lower surface of an object, while undersurface refers specifically to the flat or smooth surface underneath. The term ventral surface is often used in reference to animals and describes the bottom or belly area. Some other related words for under side include base, foundation, and support, which describe the underlying structures or foundations that support a building or object.
