The 991 was introduced in 1999 as the latest in New Balance 990 running shoe series. It became one of the most popular sneakers produced by New Balance. At the end of 2005, the 991 was discontinued and replaced by the the 992.
The look of the 991 and 992 are almost the same. The biggest difference is the design on the bottom of the shoe (the 991 has a arcing stripe of color on the bottom, the 992 does not). Both are available in multiple colors.
The 992 is available in two bottom width sizes. The bottom of the shoe is smaller for smaller sizes and bigger for bigger sizes, unlike the 991. Upgrades have also been made to the materials on the outside of the 992, and on its collar to reduce wear.
The major differences between the 991 and 992 are the materials inside the shoe. For example, the 992 has an ABZORB insert, while the 991 has a standard insert made of EVA. The ABZORB material is used for the forefoot and heel of the 991, but in the 992 these parts are made from New Balance's enhanced ABZORB SBS material. Both have a C-Cap core and an Encap midsole/heel.
Although the 991 is no longer manufactured, it can sometimes be found for sale through the Internet. The 992 itself was phased out and replaced by the 993 in 2009.