RRBS:
a variation of whole genome bisulfite sequencing that uses restriction enzyme (e.g., MspI) digestion and DNA size selection to focus the analysis on a subset of the genome with high CpG content, thus avoiding the high cost associated with WGBS. This method measures 10-15% of all CpGs in the genome at single nucleotide level.

Reference: Gu, H., et al., Preparation of reduced representation bisulfite sequencing libraries for genome-scale DNA methylation profiling. Nat Protoc, 2011. 6(4): p. 468-81.