SPXB is the first ETF that exclusively hold bonds from issuers that are included in the S&P 500. All holdings must be investment-grade, denominated in US dollars, with at least one year remaining to maturity. From the pool of eligible securities issued by S&P 500 companies, SPXB selects the 1000 most liquid issues (typically the entire pool) as characterized by highest average trading volumes. The Index is weighted by market value, and is rebalanced every month.