When changing the indent of the current line, preserve as much of the indent structure as possible. Normally the indent is replaced by a series of tabs followed by spaces as required (unless |'expandtab'| is enabled, in which case only spaces are used). Enabling this option means the indent will preserve as many existing characters as possible for indenting, and only add additional tabs or spaces as required. 'expandtab' does not apply to the preserved white space, a Tab remains a Tab. NOTE: When using ">>" multiple times the resulting indent is a mix of tabs and spaces. You might not like this. NOTE: This option is reset when 'compatible' is set. Also see 'copyindent'. Use |:retab| to clean up white space.