What a connection chart shows
Human Design reads two people together by laying one bodygraph over the other. The calculator draws Person 1's gates light (black when printed) and Person 2's in red, the way a single chart draws its Personality and Design, and a gate you both have is striped. Two things are then read from the combined chart: how each channel between you is made, and how many of the nine centers the two of you define together. Each of your own charts still matters as much as the combination, so the result also gives each person's type, strategy, inner authority and profile, and the Human Design chart calculator gives either of you the whole chart.
The four ways to meet in a channel
Jovian Archive's Human Design dictionary describes four ways two charts can meet in a channel. In an electromagnetic channel, each of you has the gate at one end and neither has both, so you complete it together; it is read as the spark between two people, attraction and repulsion alike. In companionship, you both define the same channel and share that energy in the same way, read as ease and familiarity. In dominance, one of you defines a channel and the other has neither of its gates, so the other takes that energy in through the relationship. In compromise, one of you defines a channel and the other has one of its gates, and the one with the whole channel sets how it works; the tradition calls this the hardest of the four.
Measured on 5,000 pairs of birth moments spread evenly over 1950 to 2009, 96% of pairs have at least one electromagnetic channel, 96% at least one of dominance and 97% at least one of compromise, about three of each on average. Companionship is the rare one: only 31% of pairs share even one channel, which fits the system's saying that people are drawn to what they are not.
Your centers together
The combined chart's defined centers are counted as defined against open: a 9-0 connection defines all nine between you, a 5-4 only five. On the same 5,000 pairs, 8-1 is the most common at 36%, then 9-0 at 30% and 7-2 at 24%; 6-3 takes 8%, and 5-4 or fewer about 2%. The more that is defined between two people, the more fixed and familiar the connection is read to be, and each center left open is where both of you take in the people around you. More than half of pairs, 59%, define at least one center together that neither defines alone, always through an electromagnetic channel, since every other kind of channel is already defined in one of the two charts.
How to read it
A connection chart describes mechanics, not a verdict. It shows where your energies meet consistently, where one of you carries something for both, and where you spark, but it cannot say whether two people belong together, and a hard channel is not a reason to part. The system's advice for any relationship starts with each person living by their own strategy and authority, which is why both appear in the result. Read a channel you share in the guide to the 36 channels, and each person's type in the guide to the five types.
Birth times and privacy
The Moon moves into a new gate about every ten hours, in the birth chart and in the Design, so an unknown birth time can complete, break or change the kind of a channel between you. Without a time, the calculator draws that chart for noon and says so. Both sets of birth details are calculated in your browser and never sent anywhere; with Remember my details on, the last pair is kept in this browser only, as for the synastry chart, which compares the same two births in astrology.