Early marker-based macrocolinearity studies between the grass genomes led to arranging their chromosomes into concentric 'crop circles' of synteny blocks that initially consisted of 30 rice-independent linkage groups representing the ancestral cereal genome structure. Recently, increased marker density and genome sequencing of several cereal genomes allowed the characterization of intragenomic duplications and their integration with intergenomic colinearity data to identify paleo-duplications and propose a model for the evolution of the grass genomes from a common ancestor. On the basis of these data an 'inner circle' comprising five ancestral chromosomes was defined providing a new reference for the grass chromosomes and new insights into their ancestral relationships and origin, as well as an efficient tool to design cross-genome markers for genetic studies.