One manifestation of the trend towards the strengthening of copyright protection that has been noticeable during the past two decades is the secular extension of the potential duration during which access to copyrightable materials remains legally restricted. Those restrictions carry clear implications for the current and prospective costs to readers seeking “on-line” availability of the affected content in digital form, via the Internet. This paper undertakes to quantify one aspect of these developments by providing readily understandable measures of the restrictive consequences of the successive modifications that were made in U.S. copyright laws during the second half of the twentieth century. Specifically, we present estimates of the past, present and future number of copyrighted books belonging to different publication-date “cohorts” whose entry into the public domain (and consequent accessibility in scanned on-line form) will thereby have been postponed. In some instances these deferrals of access due to legislative extensions of the duration of copyright protection are found to reach surprisingly far into the future, and to arise from the effects of interactions among the successive changes in the law that generally have gone unnoticed.