1. Mutual aspects to Saturn. Touching someone's Saturn means triggering their long-term interest/involvement/desire to commit (potentially, if you fit their Saturn profile)
As major examples:
With Sun, general commitment to you (to your basic self, to you as a whole)
With Moon, mother/father, child/parent archetype, emotional commitment and sense of belonging/protection
With Venus, need to love and be loved by the person, love growing with time, love commitment
With Mars, sexual commitment, long term sexual interest, growing with time
2. Trines. Trine energy is familiar from the start, on a psychological level, and they have a comfortable energy. They are about "sliding together", peacefully, in sync. They have a Saturnian value, because they are the aspect of "growing up together", in a continuous, flowing, familiar way, they have strong time-related attributes (continuum, durability), also their Saturnian vibe comes from the idea of "that which is familiar", another Saturnian attribute.
In the natal charts, they show how our inner (psychological) attributes grow and evolve with time - such as talents or other personal gifts/aptitudes. A synastric trine will trigger this process of evolving as a psychological unit/team (easily and naturally, as if belonging to the same family), depending on their nature (there are four types of trines: Air, Fire, Water and Earth trines). Trine energy (that familiarity) doesn't diminish with time, it has an unaltered quality, it's the same energetic print with meeting an old friend and feeling as if you parted ways yesterday.
Another long-term value of trines comes from the energy of MUTUAL TOLERANCE AND ACCEPTANCE they bring.
And finally, their contribution to long-termness resides in how trines love routine, habits, rituals, which means in a relationship, they will reflect that part of it when the two people like to relive patterns of behavior and share comfortable previously known and practiced activities together; they create an area of familiarity, memories and common habits both people can be very fond of.
3. Overlays to fixed houses and/or aspects from the other’s personal planets to fixed house rulers.
The fixed houses in the horoscope are houses 2, 5, 8 and 11. The 2nd/8th axis, corresponding to Taurus/Scorpio energy shows an area where we experience commitment to values or people, emotional devotion and stability, the desire to build fixed, stable structures in our lives. The 5th/11th axis is when we experience the true formation of stable feelings (a feeling being in fact a more complex, stabilized emotional construct), starting with the 5th house, where long-term romantic attachments become possible, and a continuum of romantic and sexual attraction under the rays of Leo energy, whilst at the level of the 11th house, we already project the couple as a team heading towards the future, integrated in the community, and this is also the house where we feel our hopes and desires in relationship are likely to be fulfilled with this partnership. Furthermore, while the 5th house is the area where we tend to “go” after another, romantically, to feel stimulated and intrigued to “conquer, seduce and romance” them, the 11th house is where we are likely to experience the reverse: feeling loved, wanted, appreciated, admired, put on a pedestal, worshiped, and the center of attention.
In my experience so far, there are significant connections between the fixed levels in a chart in long-term relationships: direct overlays (especially when the native has their own planets in fixed houses) and/or through rulership (aspects between one’s personal planets and the other’s fixed house rulers). One two is usually not enough to qualify for this marker, there will be at least several such aspects between the two charts, mutually.
For romantic/marital relationships, the main personal levels of a chart are: angles, personal love planets Sun Moon Venus and Mars, love rulers (and any planet can be a personal love ruler) and the nodes.
4. Midheaven (MC) involvement. In brief, this equals having a common future together. Conjunctions (or a bundle of other aspects) to MC; 10th house overlays; and/or MC rulers making significant aspects to the other’s chart, especially on the level of personal planets. Mutual and strong MC connections on the above levels are always present in long-term relationships.
5. Juno and Vesta archetype match. Personal aspects to asteroids Juno and Vesta on both sides and/or sign/house resonance between once’s Juno and Vesta and the other’s chart. Juno is the asteroid of marital devotion and it also describes the partner we will eventually commit to, and our own style of partnership in long-term marital-type relationships. Vesta is devotion in general, fidelity, and also persistent and consistent interest and passion for something/someone. Both asteroids show who the person is likely to commit to and to form a long-term partnership with and to experience long-term love and general devotion and faithfulness in a relationship.
Same sign conjunctions (if not by orb) seem to weigh a lot here, meaning having personal planets in the same sign as the other’s Juno and/or Vesta. But direct aspects with angles, Sun, Moon, Venus, Mars, love rulers have to be present too. The resonance comes from having similar Juno and Vesta archetypes, by mirroring, or triggering the other’s archetype, even in an indirect manner, for example one of the partners has Juno in the 1st, and the other is aspecting it with their Mars, which is a touchdown when it comes to the same energetic print. Or one of them has Vesta trine Saturn in their chart, and the other is a strong Saturnian, and if this partner also makes direct aspects to this Vesta, especially with their Saturn, they will match Vesta’s type.
Completing the other's Juno and Vesta geometry is another major part of activating Juno and Vesta archetype in a chart through synastry.
The rule with these big 5 is that they all have to be present, because they are all present, as identified markers, in long-term relationships, and Saturn remains as the most important one, it is unlikely for a relationship to last if Saturn, the lord of time, is not properly aspected on both sides.
NOTE: The 5 markers apply to a synastry which is already marital, cohesive and where the basics are already covered: richness of aspects to personal planets and love rulers on both sides, matching individual geometry and symbolism; mutual type match, and they refer to the potential for durability of an already well structured basic synastry.