Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to a @C01057@ centre during a chemical reaction or @T06446@. It is also the configurational correlation when a chemical species Xabcd is converted into the chemical species Xabce having the same @R05260@. The configurational change when a chemical species Xabcd (where X is typically carbon), having a tetrahedral arrangement of bonds to X, is converted into the chemical species Xabce having the opposite @R05260@ (or when it undergoes an @I02940@ in which Xabcd of opposite configuration is produced) is called a Walden @I03146@ or @I03146@ of configuration. The occurrence of a Walden @I03146@ during a chemical @T06446@ is sometimes indicated in the chemical equation by the symbol shown below in place of a simple arrow pointing from reactants to products.