It is widely acknowledged that certain feature combinations are more likely to occur than others. For example, the feature of nasality is much more likely to appear on segments that are voiced than on segments that are voiceless (see discussion below). Several properties of such combinatorial restrictions are important, including the following: (i) the motivation or source of such restrictions, (ii) their cross-linguistic variability, (iii) their language -internal strength, (iv) the manners in which they manifest themselves. This paper examines certain aspects of the phonology of nasal segments that bear on these issues. The paper focusses on the phenomenon of nasal opacity, where opacity is used to refer to the arresting of a process of f...