Covalent post-translational modifications (PTMs) provide vast indexing potential and expanded protein use. The 'histone code' hypothesis has inspired rapid advances throughout chromatin biology, and has recently been tapped for its relevance to non-histone proteins. Comprehensive analyses suggest that rather than constituting a general code, the covalent modifications of proteins (including histones) provide surfaces that are recognized by effectors that can give rise to intricate interactions and downstream events. These are reminiscent of other regulatory cascades in transcription and cell signalling.