The four following types of groups,

1. linear groups,

3. symplectic groups, and

4. unitary groups,

which were studied before more exotic types of groups (such as the sporadic groups) were discovered.

The four following types of groups,

1. linear groups,

3. symplectic groups, and

4. unitary groups,

which were studied before more exotic types of groups (such as the sporadic groups) were discovered.

Weisstein, Eric W. "Classical Groups."
From *MathWorld*--A Wolfram Web Resource. https://mathworld.wolfram.com/ClassicalGroups.html