SCHK provides core US large- and midcap stock exposure. The ETF is based on the same methodology as the Schwab 1000 Index mutual fund, which opened in 1991. The underlying index screens all eligible US common stocks for minimum liquidity and trading requirements, then selects the top 1,000 by market-cap. The portfolio is also weighted by market-cap. The index is reconstituted annually and rebalanced quarterly.