Board Size - ATZ board size is 12” wide by 9.6 deep (305mm x .244mm). This allowed many ATX chassis to accept micro ATX boards.
Mounting hole placement - Interestingly ATX holes can be mounted with Micro ATX used holes where possible.
Connector Placement - Originally AT standard required mounting on any additional connectors, these additional could be video, printer or any other connections such as network. Another option apart from mounting was use of custom-cut holes. This was because it had only a single keyboard port. This specification would provide recommendations; however, it is up to the judgment of the designer to allocate the exact locations of the various connectors. Read more on USPacket.