Ask an experienced astrologer what keeps couples together and many will mention Saturn before Venus. Saturn contacts between two charts are often described as the "glue" of long relationships. They can also be the reason one person feels criticized or held back, so it's worth reading them closely.
Saturn represents structure, time, and responsibility. When one person's Saturn makes a close aspect to the other's Sun, Moon, Venus, or Ascendant, the relationship tends to feel serious early on. Any synastry chart calculator will list these aspects; look for orbs within about three degrees.
Saturn to the Sun
The Saturn person often acts as mentor or anchor. The Sun person may feel supported in becoming more grounded, or limited in how freely they can shine. Conjunctions and trines tend toward the first experience, squares toward the second.
Saturn to the Moon
This is the most sensitive Saturn contact. It can bring a deep sense of stability, but the Moon person may feel their feelings are being judged or rationed. It often improves with age and experience on both sides.
Saturn to Venus
Saturn-Venus contacts are classic signs of commitment and loyalty. They also tend to slow romance down. The relationship may start as friendship or take a long time to become official.
Saturn to the Ascendant
When someone's Saturn sits on your Ascendant, they may seem to see you as more serious, older, or more capable than you feel. That can be flattering or exhausting depending on the day.
Reading Saturn fairly
Saturn contacts aren't warnings. A relationship with no Saturn ties at all can feel exciting but short-lived. What matters is the balance: Saturn plus warm Moon and Venus contacts often describes a relationship that is both affectionate and dependable. Saturn with no softening aspects is more likely to feel like a job.
It also helps to know where Saturn falls in each natal chart before comparing. Astrology Geeks has free calculators if you need both birth charts first.