Earlier low profile PCs used a form called LPX or LPM. These had a riser card to put the peripheral slots on their side. There was no official standard for the design; many PCs in this formfactor were brandnames that used their own proprietary layouts. Occasionally you would see a double sided riser with as many as five slots, however single sided with three was more common. LPX/LPM designs were among the first to integrate many devices such as sound, video and network onto the board since there was rarely enough room in the case for many add-on cards, especially larger ones. NLX was designed by Intel to replace LPX/LPM and was introduced in 1997. However it has essentially disappeared to be replaced by proprietary designs or Low Profile ATX chassis based on micro ATX and other form factors.
Refer to the previous articles showing Compaq low profile desktops to see an example of a proprietary implementation, in this case using a riser card for the slots.
These days low profile clones and some brandnames are produced using MicroATX boards put into a case which has a low profile backplane. A brandname example is the IBM Netvista small form factor desktop PCs in the era of the Celeron 766. A clone case example is the Foxconn DH153 chassis. The case backplane is lower than the standard PCI card and only accepts low profile cards that are fitted with a bracket made in this size. Often, manufacturers produce cards in the low profile form factor and supply brackets for both backplane sizes with the card; however it is necessary to shop around to find them. The Compaq D5S low profile desktop shown in a recent article in this blog was a hybrid; it uses a low profile graphics card in an AGP slot as well as providing two full height PCI slots on a riser. However it's not clear whether the AGP card was a standard low profile design or a proprietary unit.
The picture below shows an ATX board and micro ATX alongside each other.
The layouts are generally similar. The ATX at left has onboard sound while the uATX at right has onboard graphics and network interface as well. The CPUs are a Pentium 166 MMX and Celeron 1000 (100 MHz memory bus) respectively. The chipsets are Intel 430TX and SiS 630 while the models are Asus TX-97X and Asus TUSI-M. The main reduction in size comes from cutting the number of peripheral slots; three is enough these days with onboard integration and USB etc. The other major difference between the boards is the elimination of bus and CPU clock speed and voltage jumpers on the later boards, the settings now being done in the BIOS.
