Part 4 of Volume 24 in the series completes the review of fused pyrimidines in which the second ring is six-membered and contains one or more of the elements of nitrogen, oxygen or sulfur. In keeping with the tradition established by the three previous parts of Volume 24, the text provides a critical survey of synthetic methods and reactions of each class of compound. This is followed by tables of individual compounds containing practical information such as melting points and spectral data.