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Despite a growing and changing assortment of dolls and doll lines, throughout the years since Monster High take-off, there is one consistent and recurring line. This line, recognizeable by assortment number N2851, is without a name, on account of it containg the 'default' dolls and thus not needing a specifier. This "main" series of dolls primarily represents the characters as they commonly appear in fiction. They are released in waves, sets of 7 à 8 dolls that in marketing, release dates, and diary content share significant common ground.

Between the first wave, the Basic line, and the second wave, the School's Out line, five characters are shared. To be able to distinguish between them, the fandom thought up the term School's Out for the second series, while reserving the term Basic for the first line and the non-repeated characters of School's Out. However, this practice was and is not consistent, which has led the wiki to define Basic and School's Out solely as identifiers of the respective waves regardless whether a character is repeated or not. The same principle goes for each subsequent wave.

So far, five waves have been released in the main line, introducing the following dolls:

The currently active wave is the fourth one, with Robecca Steam, Venus McFlytrap, and Invisi Billy all scheduled for release within 2012. The line has yet to receive an announcement on a customary 2-pack.

C.A. Cupid's release is the odd one, and represents a line entirely on its own. In June 2012, Mattel re-released a random assortment of Basic and School's Out dolls, which already was unexpected because it was believed that the Basic series was definitely over. But what was even more unexpected was the inclusion of a C.A. Cupid doll, almost identical to her Sweet 1600 release, in main line packaging - diary included. Since she is the only new main doll of the assortment, she alone makes up a series. And at that, a series released in the middle of another.

The Basic Ghoulia Yelps and Holt Hyde dolls technically also belong to the School's Out series, on account that they have been re-released in School's Out packaging and were marketed as School's Out dolls in June 2011. This was probably done because they had not been out for that long when School's Out hit the shelves. However, since both dolls are the same and come with the same accessories, the wiki only lists them as Basic for management purposes.