Dyad
A dyad is a group containing two people. Because an individual does not constitute a group, the dyad is the smallest group possible.
The dyad is a particularly fragile group in that it is easily disbanded. In a dyad, if only one person is unwilling to continue with the relationship, the dyad falls apart. Thus, a dyad relies heavily on the members' levels of commitment. There are other ways for a dyad to end as well, such as the death of one member or the joining of another (as in a love triangle).
Dyads are also characterized by a strong level of intimacy. When the dyad is interacting, both members of the group are involved. Moreover, each member in the dyad must focus on the other—there are no other group members to draw attention away.
Triad
The triad, consisting of three members, is the second smallest group. Triads are fundamentally different from dyads. The relationship between a mother, father, and child is fundamentally different from the relationship of a childless married couple. A supervisor–worker–apprentice relationship is fundamentally different from the relationship of worker and apprentice. The love triangle is fundamentally different from a couple's relationship.
The differences arise from the characteristics of a triad. The triad is special in that any one member's attention may be drawn away from another member to the third. This has a number of implications in the net of relationships among all of the members in the group.
Like the dyad, the triad is also unstable but for a different reason. A triad has three person-to-person relationships. These relationships may vary in strength. As a result, two group members with the stronger bond may be able to exert control on or exclude the other member. However, the triad can also lead to higher stability, such as when a single member helps the other two establish a stronger bond. That sometimes is the case in marriages when a child is born.
Small Groups
Sociologists use the term small group for any group with more than three members. Small groups tend to have higher stability than dyads and triads because it takes a larger number of members experiencing problems within the group to reduce the group by a significant amount.
For example, in a small group with 10 members, a few members can have falling outs, yet the group as a whole will not be greatly affected. The other seven or so members with stable relationships can hold the group together. As a small group increases in members, it increases in stability.
Although a small group increases in stability as it grows in members, it also decreases in intimacy. As stated before, the addition of new members to a group draws attention away from other members. When new members are added, the number of relationships within a group increases. The lack of intimacy and the complexity of relationships in small groups causes a structure to form. Small groups of considerable size tend to develop a level of formality and a hierarchy. Members in small groups begin to take on statuses and roles that are specific to that group to create order and organization.