LPX is a motherboard form factor originally developed by Western Digital when it was making motherboards that was used in the late 1980s and throughout the 1990s. An LPX motherboard is 9" wide x 13" deep, uses a riser card, and has different placement of the video, parallel, serial, and PS/2 ports when compared to other motherboards. Below is a picture of a Packard Bell LPX motherboard, as can be seen this motherboard has one large brown slot for the riser card. Using a riser card allowed the expansion cards to be installed parallel to the motherboard, which is why the LP in LPX stands for "Low Profile." The low profile allowed computers using this motherboard to be much slimmer than computers using a Baby-AT motherboard.
The LPX motherboard was not the only motherboard to have a riser card. The NLX motherboard also has a riser card, however, the NLX motherboard actually plugged into the riser card instead of having the riser card on the motherboard.