Alternatively, you can space the seeds 6 to 10 inches (15.24 to 25.4 cm) apart and thin your seedlings out once they reach 4 inches (10 cm) tall. At that point, you should consider thinning them out so that they are 18 inches (45.72 cm) apart.
Volume - Arcy Art Original Oil Paintings Art Dictionary. Volume is the size, bulk and dimension of a particluar object. It is an object that is usually thick, nontransparent and exists solidly and three-dimensionally in space and it is not flat. In a painting a volume is represented with light and shade and usually in spatial situation to make the nonflatness clear.
Shapes are flat. If you take a shape and give it three dimensions, it has volume. A three-dimensional form has volume. Volume (three-dimensionality) can be simulated in a two-dimensional work (like a painting). This self portrait by Rembrandt is an example of simulated, or implied volume.