It was a tale of two months, but the market advance that began in March rolled on through July, with the Dow Jones Industrial Average ($INDU) gaining 9.1 percent to close above 9,170 -- the best month for the index since October 2002, when markets bottomed following the bursting of the tech bubble. The Nasdaq ($COMPX) and S&P 500 ($INX) also rose more than seven percent each.
Heavy equipment maker Caterpillar's (CAT) 32 percent gain helped power the index higher, as the company handily beat estimates and anticipated a recovery by the end of this year. Diversified manufacturer 3M Co. (MMM) added 17 percent in July, and IBM (IBM) rose 12 percent, both of which also helped the Dow Jones Industrial Average significantly because of its weighting methodology. The Dow is price-weighted, with companies that have higher share prices making up a larger portion of the index; contrasting that is a capitalization-weighted index like the S&P 500, where a company's market cap determines its representation.