Throughout the Indonesian Archipelago, an array of colloquial varieties of Indonesian are spoken, differing significantly - sometimes to the point of mutual non-intelligibility - from each other as well as from Standard Indonesian. A still widely held view holds that that such varieties are merely "debased" or "corrupted" forms of the standard language, and hence not worthy of serious investigation. Most linguists working on Indonesian today now reject this view; however, some of them still adhere to a more sophisticated version of the same prejudice, in accordance with which colloquial varieties of Indonesian do not constitute bona fide language varieties, but, rather, are products of linguistic performance, the results of various kinds of mixing of Standard Indonesian with assorted local languages.

This paper argues that such a characterization of colloquial varieties of Indonesian is inappropriate and should be rejected. Although the logic of the argument requires that it be made, on an individual basis, for each and every variety of Indonesian, for practical reasons this paper limits its attention to a single colloquial variety of Indonesian, that spoken in the province of Riau in East-central Sumatra. In this paper, it is shown that the structure of Riau Indonesian cannot be accounted for in terms of a straightforward mixture of Standard Indonesian on the one hand, and the local substrate languages, dialects of Malay and Minangkabau, on the other. In support of this claim, three distinct arguments are presented.

Argument 1: Indeed, most of the structural features of Riau Indonesian are shared with either Standard Indonesian and/or with one or more of the local substrate languages. However, if Riau Indonesian were an automatic performance-based mixture of these language varieties, then there should exist general principles governing the choice of forms, determining when a Standard Indonesian form should be used, and when instead a form from one of the local languages should be used - and which language it should be from. However, even a cursory inspection of the data shows that no such general principles exist: whether a given feature of Riau Indonesian comes from Standard Indonesian, from Malay and/or from Minangkabau is an arbitrary and unpredictable property of Riau Indonesian, which must therefore be represented in an autonomous grammar of that variety of colloquial Indonesian.

Argument 2: Alongside the above, there exists a small residue of features of Riau Indonesian which can be attributed neither to Standard Indonesian nor to any dialects of Malay and Minangkabau. These features show that Riau Indonesian is more than just a mixture of superstrate and substrate, more than just the sum of its wound-be parts, but, instead, is a language in its own right, with its own unique grammatical system.

Argument 3: The structural features of Riau Indonesian can be shown to form part of a dialect chain constituted by other similar colloquial varieties of Indonesian throughout the archipelago. In order to make this point, over 100 dialect maps are presented, showing the distribution of various linguistic features across Sumatra, as well as in neighboring regions of peninsular Malaysia, Borneo and Java. These maps show that whether Riau Indonesian possesses a certain linguistic feature is in some cases predictable not by the properties of its superstrate and substrate languages but rather by the properties of other colloquial varieties of Indonesian spoken in neighboring regions. Thus, Riau Indonesian and its neighboring varieties exhibit geographical patterning just like ordinary dialects in classical dialectology.