Popular tetrad bases include orthonormal tetrads and null tetrads. The index notation for tetrads is explained in tetrad (index notation). The paired chromosomes are called bivalent or tetrad chromosomes. A bivalent is one pair of chromosomes (sister chromatids) in a tetrad. The spores may remain attached to each other in a tetrad or separate after meiosis. Here, four guanine bases, known as a guanine tetrad, form a flat plate. At this point a tetrad of the chromosomes has formed known as a bivalent. The trophozoites moult into merozoites, which have a tetrad structure coined a Maltese-cross form. The plants demonstrate simultaneous microsporogenesis, the microspores are tetrad, tetrahedral, or isobilateral. Trophozoites appearing in a tetrad formation within an erythrocyte are also indicative of "Babesia". The tetrad an analogical, simultaneous, four-fold pattern of transformation. The scolex also has four suckers, or a tetrad. Marshall McLuhan's tetrad of media effects. Such a quadruple is a harmonic tetrad. So is the Orwell tetrad, C-F-D-G in its various inversions.