There was the BTX standard, which failed to catch on. Most PCs had only two fans: It used the same fan as CPU fan and main intake fan, and then the PSU fan. It

There was the BTX standard, which failed to catch on. Most PCs had only two fans: It used the same fan as CPU fan and main intake fan, and then the PSU fan. It had pure horizontal airflow, quite similar to earlier aluminium Mac Pros. However, the CPU fan and cooler (including duct) had to be a fixed size. Too large for small form factor systems, and too small if you needed more cooling. ATX/mATX/mITX was cheaper, more flexible and hade more momentum.