Dialectology is often defined plainly as the study of one or more synchronic non-standard language systems (regional dialects, sometimes also regiolects) which form part of a more encompassing set of varieties, usually called a diasystem. The term ‘diasystem’ stands for the complete set of varieties (diachronic as well as diatopic-synchronic) supposed to derive from one ancestor; often a restriction is added in the sense that such a diasystem also presupposes a common standard language covering all synchronic varieties.