The lines of text in some of the diff output formats are preceded by one or two characters that indicate whether the text is inserted, deleted, or changed. The addition of those characters can cause tabs to move to the next tab stop, throwing off the alignment of columns in the line. GNU diff provides two ways to make tab-aligned columns line up correctly.
The first way is to have diff convert all tabs into the correct number of spaces before outputting them; select this method with the --expand-tabs (-t) option. To use this form of output with patch, you must give patch the -l or --ignore-white-space option (see Changed White Space, for more information). diff normally assumes that tab stops are set every 8 print columns, but this can be altered by the --tabsize=columns option.
The other method for making tabs line up correctly is to add a tab character instead of a space after the indicator character at the beginning of the line. This ensures that all following tab characters are in the same position relative to tab stops that they were in the original files, so that the output is aligned correctly. Its disadvantage is that it can make long lines too long to fit on one line of the screen or the paper. It also does not work with the unified output format, which does not have a space character after the change type indicator character. Select this method with the -T or --initial-tab option.