Ultrastructure (or ultra-structure) is the detailed structure of a biological specimen, such as a cell, tissue, or organ, that can be observed by electron microscopy. It refers in general to the study of cellular structures that are too small to be seen with an optical microscope.

Ultrastructure, along with molecular phylogeny, has often been a reliable (that is phylogenetic) way of classifying organisms.

Ultra-Structure is also the name given to a notational system for representing complex rules.

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