ATX case is not used for all but it is still used for most.
ATX is a form factor. It does not dictate a specific chipset; anything from the now ancient 440BX to a Q57 can be used in an ATX system.
The official ATX website offers detailed information about ATX software. The ATX software is used to aid in tax filing and more information about its usage can also be found on its official website.
There's actually three: ATX, Mini ATX, and Micro ATX. The difference is only in size. Mini is about half the size, Micro is 1/3 But of course with the size, you sacrifice extra ports, usually PCI expansion ports.
usually any motherboard that has a form factor above mini or micro atx (atx, extended atx, super atx) will have atleast 4 if not more expansion slots available by default and normally an atx (and possibly a extended atx) motherboard should fit in a full tower case (a super atx motherboard will more than likely need a super tower)
The three form factors for desktop system boards are: ATX, and BTX and NLX
ATX, MicroATX, Flex ATX, BTX, and NLX, In that order.
There are not really any advantages with a ATX over a Micro ATX just Micro ATX is smaller and ATX is larger
Most ATX motherboards offer a choice of several similar processors. The manufacturer decides which processors can used by a specific ATX motherboard. ATX is a physical and electrical specification. ATX does not define a specific central processor, so theoretically an ATX motherboard could be created for any type or brand of processor that can function within the specification. Common ATX motherboards use either an Intel processor or an AMD processor.
ATX, Actually todays most common motherboard form factor used is MicroATX
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