The `-y' or `--side-by-side' option selects side by side format. Because side by side output lines contain two input lines, they are wider than usual. They are normally 130 columns, which can fit onto a traditional printer line. You can set the length of output lines with the `-W columns' or `--width=columns' option. The output line is split into two halves of equal length, separated by a small gutter to mark differences; the right half is aligned to a tab stop so that tabs line up. Input lines that are too long to fit in half of an output line are truncated for output.
The `--left-column' option prints only the left column of two common lines. The `--suppress-common-lines' option suppresses common lines entirely.