Updated project metadata. Chromatin states must be stably maintained during cell proliferation to uphold cellular identity and genome integrity. Inheritance of histone modifications across cell division is thought to be central in this process. However, the histone modification landscape is challenged by the incorporation of new unmodified histones during each cell cycle and the principles that govern heritability remain poorly defined. Here, we take a quantitative approach and develop a reusable computational model that describes propagation of K27 and K36 methylation states. We measure combinatorial K27 and K36 methylation patterns by quantitative mass spectrometry on subsequent generations of histones in the presence and absence of enzymatic inhibition. Our modelling rejects active global demethylation and invoke the existence of 8 domains defined by distinct methylation endpoints. We find that K27me3 on pre- existing histones stimulates the rate of de novo K27me3 establishment, supporting a read-write mechanism in timely chromatin restoration. Finally, we provide a detailed, quantitative picture of the mutual antagonism between K27 and K37 methylation, and propose that this antagonism enhance the stability of epigenetic states across cell division.