What slim factor is similar to ATX?

The slim factor similar to ATX is the Micro ATX (mATX) form factor. Micro ATX boards are smaller than standard ATX boards, typically measuring 244 x 244 mm, while still offering support for multiple expansion slots and components. This makes them a popular choice for compact systems that require a balance between size and expandability. Additionally, there are Mini ITX boards, which are even smaller, that also share some design similarities with ATX.