AS/400
Also known as: AS400, Application System/400
IBM's original official brand spelling for the Application System/400 platform introduced in 1988. AS400 is the later slashless shorthand and search form.
AS/400 is the original IBM brand-correct styling for the Application System/400 platform introduced in 1988. IBM used the slash in product branding, documentation, and launch materials. Over time, users often dropped the slash when typing quickly, writing job titles, creating URLs, or searching online, which is why AS400 became the dominant shorthand. On this site, both forms are used intentionally: AS/400 for the original official brand spelling, and AS400 for the search-heavy colloquial form most users type today.