The opposite of "is vertical" is "is horizontal." A vertical line or object stands straight up and down, perpendicular to the ground or another surface, while a horizontal line or object lies flat and level, parallel to the ground. Something that is vertical may also be referred to as upright, while something that is horizontal may be called level or flat. In some contexts, other antonyms for vertical may be used, such as slanted, angled, or diagonal, depending on the specific orientation of the object or line in question. Understanding these antonyms can be useful for describing the shape, position, or movement of objects in various settings.