Varve chronology is the use of varve sequences to establish time lines in sedimentary sequences and for correlation. The advantage that varves have over other sediments is that they have tremendous precision of a year and in some cases down to the level of seasonal layers within a varve if intra-annual stratigraphy shows a consistent separation of seasonal features. Correlation of glacial varve records from place to place is generally based on the matching of the pattern of varve thickness change and not absolute thickness, which varies widely for a single varve year across a lake or region. In addition, correlations can sometimes be established by matching basin-wide lithologic changes in varve sequences if they represent isochronous events.