A K-1 is a tax document used to report share of profits and losses from interests in limited partnerships. Due to their legal structuring, many ETPs (exchange traded products) issue K-1’s instead of the more common 1099 tax form. Additional information on K-1’s, along with a list of ETPs which issue K-1’s, can be found in this brief article from Yahoo Finance:

ETF Tax Tutorial: Complete List Of ETFs That Issue A K-1

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