Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to a @[email protected] centre during a chemical reaction or @[email protected] It is also the configurational correlation when a chemical species Xabcd is converted into the chemical species Xabce having the same @[email protected] 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 @[email protected] (or when it undergoes an @[email protected] in which Xabcd of opposite configuration is produced) is called a Walden @[email protected] or @[email protected] of configuration. The occurrence of a Walden @[email protected] during a chemical @[email protected] is sometimes indicated in the chemical equation by the symbol shown below in place of a simple arrow pointing from reactants to products.