Is the application of more than one number system in a particular culture necessarily an indication of not having abstracted a general concept of number? Does this mean that specific number systems for certain objects are cognitively deficient? The opposite is the case with the traditional number systems in Tongan, where a consistent decimal system is supplemented by diverging systems for certain objects, in which 20 seems to play a special role. Based on an analysis of their linguistic, historical and cultural context, we will show that the supplementary systems did not precede the general system, but were rather derived from it. Especially when notation is lacking, having such supplementary systems can even yield cognitive advantages. In using larger counting units, they both abbreviate counting and expand the limits of the general system, thus facilitating the cognitive task of mental arithmetic.