System for modifying properties of an article of footwear

An article of footwear that is modifiable to have one of a first width and a second width, said footwear including an upper, an outsole that is attached to said upper, and a foot-supporting midsole that is removable from said footwear, said midsole comprising:

• a frame portion that underlies a substantial portion of a foot that is received by said footwear, said frame portion defining an aperture that extends from an upper surface of said frame portion to a lower surface of said frame portion;
• a first insert portion and a second insert portion that have different widths, each said insert portion having an upper surface and a lower surface that align with said upper and lower surfaces of said frame portion, said aperture being configured to receive one of said first insert portion and said second insert portion, said footwear having first width when said first insert portion is removably received by said aperture, and said footwear having said second width when said second insert portion is received by said aperture;
• wherein each said insert portion comprises a longitudinal spine and a laterally extending key, said key being flush with said upper surfaces, and said aperture comprises a depression flush with said upper surfaces shaped to removably receive said key to restrain said insert portion in said aperture of said frame portion.