Synastry is the chart that shows the chemistry between two people, how their individual planets interact with eachother. It shows the opportunities however it does not necessarily show the result or final product of two people being in a relationship. For this you need to look at the Composite chart to see how the combination of both individuals turns out in practice.

I have a whole page devoted to Synastry, regards aspects, house overlays, etc etc.
But i have always found the Asteroids to be rather fascinating in terms of seeing deeper layers to what brings two people together.

Asteroids to use in Comparison charts

1) Alma (390)
Often in tight conjunction in the charts of soulmates / twinflames. Alma means soul. When it conjuncts one of the angles, it shows a strong soul connection.
Conjunction to POF, Sun, Moon, Venus or Vertex shows a strong connection.

2) Eros (433)
Sexual attraction, especially strong when it’s conjunct towards Mars, Lilith or ASC.

3) Juno (3)
Juno means commitment and is also a significator of the spouse. Often in conjunction to the Sun, Moon Or Venus in the chart of married couples.

4) Union (1585)
An asteroid which is used in predicting marriages and the merging of two people.
Conjunction with Moon, Sun, Vertex or Venus shows a strong connection.

5) Destinn (6583)
(Synastry) When in a tight conjunction to a planet of another person, it shows that person to be of (big) importance within your life. The person may have a significant influence on you and may help you to reach your goals and destiny in life. The house & sign placement shows the destiny of the couple.

6) Amor (1221)
It shows a state of unconditional loving. Especially in tight conjunction to an angle or one of the other love objects may show a real soul connection. Strongly felt when it’s conjunct to one of the angles, Moon, Sun, Venus or Union.

Asteroids are only useful after a full reading of a synastry chart without the asteroids, since they only act as extra flavor to the whole picture. They help in understanding what’s going on more.

Composite Ascendant : Type of couple

Aries : Active, self-sufficient, confident, impatient. Compromise is needed

Taurus : Traditional, practical, solid, longterm thinking. Variety is needed

Gemini : Mental, instinctive, restless, adaptable. Intimacy is needed

Cancer : Emotional, Security, comfort. Communication is needed

Leo : Confident, compassionate, Lower pride is needed

Virgo : Practical, detail-oriented. Seeing the big picture is needed

Libra : Compromising, loving, sociable. Balanced compromising.

Scorpio : Intense, transformative, sexual. Objectivity is needed

Sagittarius : Adventurous, independent. Balance is needed

Capricorn : Focused, structured, age difference

Aquarius : Independent, different.

Pisces : Dreamy, spiritual, sacrificial, sense of reality is needed

The sun and placement
In the Composite chart the Sun and the ASC are most important to look at in terms of understanding why the relationship exists in the first place.
Sun shows what brought you together. The house & sign placement gives more insight.

1st house : During sports, sudden meeting, mutual project
Aries : Outgoing, lively, impulsive, competitive

2nd house : Places related to money & possessions,
Taurus : Slow, steady, sensual, physical

3rd house : Through messaging, mutual friends, internet
Gemini : Loud, funloving

4th house : In a house, at home, in a calm environment
Cancer : Sensitive, emotional, nurturing

5th house : At a party
Leo : Outgoing, creative

6th house : In your daily routines, at work
Virgo : Service, cooperation, helping eachother, structure

7th house : Mutual acquaintances, in a group of people
Libra : Partnership, harmony, compromising

8th house : Secret meeting
Scorpio : transformation, depths of life

9th house : At a party, on the go, at university
Sagittarius : Expanding perspectives, spirituality, optimism

10th house : At work, business environments, a meeting
Capricorn : Planning, structure, determination, teamwork

11th house : In a community, online, unusual meeting, behind the scenes
Aquarius : Freedom, selfexpression, its a less emotional/intense placement

12th house : Secret meeting, during rough times, institution
Pisces : Private, artistic, spiritual, sensual

7th house
The planets that are placed in the Composite 7th house, shows what’s needed to make the relationship work.

Sun : Need for balance in ego’s inorder to make it work. It’s a good placement for marriage, but it can quickly evolve into becoming open enemies if you are not willing to compromise and listen to eachother.

Moon : Emotional involvement is a strong theme. Giving security to eachother is needed aswell as balancing the emotions of both people.

Mercury : Equally sharing is a theme. Communication is strong, but also the point that needs to be balanced inorder for it to work.

Venus : A very good placement for all love relationships aswell as business related relationships. It’s important to find balance in giving & receiving. Balance the mutual need for compromising to much.

Mars : Balance of the energy put into the relationship is needed. with this placement you are both intensely involved. Harmonious cooperation and mutual input is needed.

Jupiter : Mutual enhancement. This is a beneficial placement for any type of relationship.

Saturn : This creates commitment and durability. However mutual respect needs to be balanced inorder for it to work. See eachother as equals.

Uranus : A need to be unconventional together. LDR or open relationship. Freedom is a big theme so that both individuals can grow

Neptune : Creates difficulty in seeing the relationship as it really is, patience and mutual respect is needed. Be open with eachother.

Pluto : A very intense placement which can stir up power conflicts and a need to possess eachother. Try to balance these energies, treat eachother as equals.

Hard aspects

Neptune : Idealistic start, sour ending