Transparency panel overview
You use the Transparency panel (Window >
Transparency) to specify the opacity and blending mode of objects,
to create opacity masks, or to knock out a portion of one object
with the overlying portion of a transparent object.
Show all options in the Transparency panel
Choose
Show Options from the panel menu.
Show a thumbnail of the selected object in the Transparency panel
Choose
Show Thumbnail from the panel menu. Or, click the double triangle
on the panel’s tab to cycle through the display sizes.