大般涅槃經義記 4.88

4.88

就超聲中具有九字，取前四字歷配餘字入十二音復作四章，通前合為十二章矣。

Within the remaining sounds there are nine letters; taking the first four letters and combining them successively with the remaining letters into the twelve sounds makes another four charts, which together with the preceding makes twelve charts in all.