A fully transparent object is one that is completely see-through, like a clean, untinted window. At the other end of the scale, a fully opaque object is one that you cannot see through at all. Partial transparency (also called partial opacity ) allows an object to be made partially transparent, like a tinted lens. In common usage, applying transparency in Illustrator really means applying partial transparency, a filter that alters the density or color of an object.
In Illustrator, transparency is applied in degrees, ranging from 1 percent (almost completely opaque) to 99 percent (almost completely invisible). An object with 0-percent transparency is opaque, and an object with 100-percent transparency is invisible.
Transparency is used for objects that cover other objects. For instance, a tinted plastic sheet in Figure 17-1 tints the fruits it covers. However, the pricing signs are on top of the transparency object, and are therefore not affected.
Objects with partial transparency affect objects stacked below them. An object stacked beneath a transparent object will be partially visible.