The complexity of the Indian calendar system is not just in the plethora of available calendar, but also in the manner in which they link up with one another. A principal point of linkage of most Indian calendars is their name of the months, As we shall see, the similar set of month names are used in more than one calendar. We aim to formulate the rules, determining the name of months. Our motivation is not just taxonomic, month names, as we shall see, are critical for understanding the Indian calendar system.

The set of month names named after nakshatras is used by both solar and lunisolar calendars, adding to the complexity of the Indian calendar system. Indeed, as we shall see, this type should actually called months initially named after nakshatras; there has been an infusion of solar rules into an essentially lunar convention. The motion of the moon is related to lunar months.

Let us then, first consider the original rule. Saha and Lahiri mention that Pakshas or fortnights were differentiated based on the nakshatra. That is to say, if a particular full moon occurs near, say, the lunar asterism, Visakha, the full moon would be called as Vaisakha Pumimasi, and the month would be Vaisakha. The earliest lunisolar months, then, were pumimanta, that is, the name of the full moon corresponded to the name of the month. Of course, the full moon occurs at all nakshatras. Fifteen were taken into account for the naming of the month, spaced more or less equally. We thus have the following set of names along with their respective nakshatras: