Sagittarius
What is already familiar, instinctive, and ready to be carried forward.
Capricorn
What is arriving, stretching the old shape, and asking for a new response.
A cusp is best understood as a threshold image: Sagittarius's fire mutable quality meets Capricorn's earth cardinal quality. The Sun is still in one sign at a time, while the neighboring symbolism can describe the season around it.
Sagittarius brings
Sagittarius is ruled by Jupiter and speaks through fire mutable movement. Its familiar lesson is about the explorer.
Capricorn arrives
Capricorn is ruled by Saturn and changes the texture toward earth cardinal expression. Its invitation is to practice the elder.
A boundary, not a loophole
If you were born near a sign boundary, use an accurate birth time and place when a precise chart matters. The cusp language can be a useful metaphor, but it should not replace the actual Sun sign.