Types

Everyone uses all eight functions, but a type leads with two: one that judges and one that perceives, one deep and one quick. Introverted (i) functions go deep and collect; extraverted (e) ones are quick and interact. Thinking (T) and sensing (S) are hard, feeling (F) and intuition (N) soft. T and F judge, with duty and survival in mind; S and N perceive, which outwardly means play and probing.

The four letters say the same: INTP leads with introverted thinking (Ti), then extraverted intuition (Ne). Socionics calls that type LII.

Functions

Ti · Introverted thinking: Deep, hard judgement: working out on your own exactly how things fit together.
Te · Extraverted thinking: Quick, hard judgement: deciding, organising and getting done what works.
Fi · Introverted feeling: Deep, soft judgement: knowing inwardly what you feel, value and stay loyal to.
Fe · Extraverted feeling: Quick, soft judgement: showing feelings and tending the mood between people.
Si · Introverted sensing: Deep, hard perception: taking in concrete detail slowly and fully.
Se · Extraverted sensing: Quick, hard perception: playing with and probing the physical world, right now.
Ni · Introverted intuition: Deep, soft perception: gathering impressions until their meaning comes together.
Ne · Extraverted intuition: Quick, soft perception: playing with and probing ideas and possibilities.

The 16 types

ISTJ · SLI: Leads with Si, then Te.
ISFJ · SEI: Leads with Si, then Fe.
INFJ · IEI: Leads with Ni, then Fe.
INTJ · ILI: Leads with Ni, then Te.
ISTP · LSI: Leads with Ti, then Se.
ISFP · ESI: Leads with Fi, then Se.
INFP · EII: Leads with Fi, then Ne.
INTP · LII: Leads with Ti, then Ne.
ESTP · SLE: Leads with Se, then Ti.
ESFP · SEE: Leads with Se, then Fi.
ENFP · IEE: Leads with Ne, then Fi.
ENTP · ILE: Leads with Ne, then Ti.
ESTJ · LSE: Leads with Te, then Si.
ESFJ · ESE: Leads with Fe, then Si.
ENFJ · EIE: Leads with Fe, then Ni.
ENTJ · LIE: Leads with Te, then Ni.

Relations

How two types tend to get along, by where one's two leading functions fall among the other's eight, with socionics' names. They are tendencies, not verdicts.

Identity: The same type: you understand each other easily, but share the same blind spots.
Mirror: The same two functions, led the other way round: you see the same things from a slightly different angle, good for discussion, with friction over details.
Duality: The same functions in opposite order: likely the most complementary, each supplying what the other lacks.
Activity: Close in values and lively together: energising in doses, tiring when it lasts.
Semi-duality: Half of duality: attraction and some completion, but often a sense that something is missing.
Mirage: Easy, relaxed company, but you may drift along without pushing each other anywhere.
Business: You work in similar ways but judge by different measures: easy to cooperate, easy to misunderstand.
Kindred: The same leading function with different methods: shared interests, but you may judge how the other goes about them.
Quasi-identity: Alike from the outside but thinking in opposite directions: likely to challenge or misunderstand each other.
Contrary: The same functions facing the other way (say Ti and Te): likely to challenge or wear each other down, though you can sharpen each other.
Super-ego: Make or break: likely distant or alien, but if you get past the misunderstandings and find mutual respect, it may be very rewarding.
Conflict: Each one's strength presses on the other's weak spot: hard to be close, though care and some distance help.
Supervisor: They supervise you: they easily notice your weak spot, which can feel like pressure or criticism.
Supervisee: You supervise them: you easily notice their weak spot, and they may feel judged by you.
Benefactor: They benefit you: what comes easily to them inspires you, but it can feel one-sided.
Beneficiary: You benefit them: they look up to what comes easily to you, but it can feel one-sided.