The term is traditionally applied to any reaction that involves a change of @C01274@ (sometimes including hydrogen), and violates the so-called 'principle of minimum structural change'. According to this oversimplified principle, @CT01038@ do not isomerize in the course of a @T06446@, e.g. @S06078@, or the change of a @F02555@ of a chemical species into a different @F02555@ is not expected to involve the making or breaking of more than the minimum number of bonds required to effect that @T06446@. For example, any new substituents are expected to enter the precise positions previously occupied by displaced groups. The simplest type of @R05194@ is an @I03130@ reaction in which the product is @I03291@ with the reactant (one type of '@I03130@ @I03295@'). An example is the first step of the Claisen @R05194@: The definition of molecular @R05194@ includes changes in which there is a @M03920@ of an atom or bond (unexpected on the basis of the principle of minimum structural change), as in the reaction: where the @R05196@ can formally be represented as the '1,2-shift' of @H02904@ between adjacent carbon atoms in the @C00817@: Such migrations occur also in radicals, e.g.: The definition also includes reactions in which an @E02130@ takes up a different position from the @L03493@, with accompanying @M03920@. An example of the latter type is the 'allylic @R05194@': A distinction is made between '@I03130@ rearrangements' (or 'true molecular rearrangements') and '@I03098@ rearrangements' (or 'apparent rearrangements'). In the former case the atoms and groups that are common to a reactant and a product never separate into independent fragments during the @R05196@ (i.e. the change is @I03130@), whereas in an '@I03098@ @R05194@' a migrating group is completely free from the parent molecule and is re-attached to a different position in a subsequent step, as in the Orton reaction: