Four companies are switching places on some Standard & Poor's indices.

Vertex Pharmaceuticals and Ametek Inc. will join the S&P 500, taking the spots of Advanced Micro Devices Inc. and SAIC Inc. AMD and SAIC will take the places of Vertex and Ametek on the S&P MidCap 400.

S&P said Thursday that the moves will better align the businesses with indices more representative of their market capitalizations. Both Vertex and Ametek have market capitalizations of more than $10 billion.

The companies will switch places on Sept. 20.

S&P also announced that Capstead Mortgage Corp. will replace Rue 21 Inc. on the S&P SmallCap 600 on Sept. 19. Rue 21 is being taken private in a deal with private equity firm Apax Partners that is expected to close on that date.