Most types of layers in ArcGIS can be set to display at some level of transparency. This allows the features beneath them to be viewable, while allowing partially transparent features to be viewable as well. This is a popular technique for creating shaded relief displays, where an elevation model is displayed with partial transparency over a hillshade model or vice versa.
Transparent symbology can increase the size of ArcMap's output and may exceed the capacity of the printer or computer involved. See the link in Related Information for information about 'out of memory' errors.
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