How does one determine what constitutes substantially identical investments (in IRS eyes) so as to avoid wash sale problem when tax loss harvesting, but reinvesting in something else at the same time? Are there clear IRS or other guidelines posted somewhere to determine how much difference is enough of a difference? For example, I am thinking about selling some VG Total International (VGTSX) and buying instead VG FTSE All-World ex-US (VFWIX). VFWIX adds a little exposure to North America that is not present in VGTSX (about 6-7 percent if I recall correctly), but the remaining regional percentages are pretty similarly proportioned between the two funds. Top ten holdings have much similarity between the two (though in different rank order) and top ten holdings represent a slightly different percentage of overall in each case. VFWIX holds a larger total number of stocks than VGTSX. Is there a quantifiable metric somewhere for this is is it just that the IRS "knows it when it sees it"? Thanks for any pointers.