Aries, Cancer, Libra, Capricorn. The four signs that open the seasons, and the four points that define the year.
The cardinal signs begin at the two equinoxes and the two solstices. They are not merely first in their season; they are the four moments the solar year is defined by. 0° Aries is the March equinox, 0° Cancer the June solstice, 0° Libra the September equinox, 0° Capricorn the December solstice.
They sit 90° apart, forming a perfect cross. Any two cardinal signs are therefore square or opposite each other — never trine. The traditional restlessness of a cardinal emphasis and the traditional friction of the square are, once again, the same geometry described twice.
This is also why the four angles of a chart — Ascendant, Midheaven, Descendant, Imum Coeli — are called the cardinal points. They form the same 90° cross, rotated to the moment of birth rather than fixed to the year.