Seven Became Nine
The difference in number is deliberate. Human Design holds that the human form went through a reorganisation, dated to 1781, in which two centres split and the whole system rewired itself. The old heart chakra became two Centers, the G Center for identity and direction and the Heart Center for will. The old solar plexus chakra also became two, the Spleen for instinct and immunity and the Solar Plexus for emotion. Everything else kept a rough equivalent: the crown became the Head, the third eye became the Ajna, the throat stayed the Throat, the sacral stayed the Sacral, and the root stayed the Root.
What Each Center Does
The nine Centers are hubs where energy is transformed. Two of them apply pressure, the Head from above and the Root from below. Three are awareness centres: the Ajna for mental awareness, the Spleen for the body's instant knowing, and the Solar Plexus for emotional awareness that moves in waves. Four are motors, sources of energy: the Sacral, the Heart, the Solar Plexus and the Root. The G Center holds identity and love. The Throat is where everything is expressed and turned into action. Each Center also corresponds to a biological system, from the pineal gland at the Head to the adrenals at the Root, and each has its own page in the Centers section.
Defined Versus Undefined
Where the chakra traditions speak of centres being open, blocked or balanced, Human Design speaks of defined and undefined, and it means something more fixed. A defined Center is coloured in on your chart and behaves the same way all your life. An undefined Center is white, has no consistent way of operating, and takes on the energy of whoever is nearby. This is the single biggest practical difference between the two systems: in Human Design an undefined Center is not a fault to be corrected but a place of learning and, eventually, wisdom.
The 1781 Story
The date matters because it is when Uranus was discovered, and Human Design ties the arrival of the nine-centred form to that moment. The mind-led, seven-centred human was replaced within a few generations by a form built to be guided by the body, with a longer lifespan set by the 84-year Uranus cycle. This is why the system insists so firmly that the mind is a poor decision maker. The page on the Nine-Centered Being takes the story further.