LPX (Low Profile eXtension) is a motherboard form factor developed by Western Digital in 1987, 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 compared to other motherboards. Below is a picture of a Packard Bell LPX motherboard. As shown, 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, but it actually plugged into the riser card versus having the riser card on the motherboard.