Number of spaces that a <Tab> counts for while performing editing operations, like inserting a <Tab> or using <BS>. It "feels" like <Tab>s are being inserted, while in fact a mix of spaces and <Tab>s is used. This is useful to keep the 'ts' setting at its standard value of 8, while being able to edit like it is set to 'sts'. However, commands like "x" still work on the actual characters. When 'sts' is zero, this feature is off. When 'sts' is negative, the value of 'shiftwidth' is used. 'softtabstop' is set to 0 when the 'paste' option is set and restored when 'paste' is reset. See also ins-expandtab. When 'expandtab' is not set, the number of spaces is minimized by using <Tab>s. The 'L' flag in 'cpoptions' changes how tabs are used when 'list' is set. NOTE: This option is set to 0 when 'compatible' is set.

If Vim is compiled with the +vartabs feature then the value of 'softtabstop' will be ignored if 'varsofttabstop' is set to anything other than an empty string.

(default 0)