Covert channels have traditionally been categorized as either storage channels or timing channels[6, 7]. This paper questions this categorization, and discusses channels that cannot be clearly identified as either or timing channels, but have aspects of both. A new model of timing channels is presented, which allows for channels that have characteristics of both storage channels and timing channels, and a method is given for constructing all channels in a computer system that have timing channel characteristics. Since Kemmerer's shared resource matriz methodology [5] has the potential to detect all channels that have storage characteristics, the two methods jointly have the capability to construct all channels in a computer system. The approach to timing channels provides a justification of specific mechanisms for reducing their bandwidth which were employed in the VAX¹ Virtual Machine Monitor, as described in [3] and [2].