Definition:Internal Tangent

Definition

Let $C_1$ and $C_2$ be circles embedded in the plane such that:

$C_1$ and $C_2$ do not intersect

one is not inside the other.

Consider the $4$ common tangents to $C_1$ and $C_2$.

The internal tangents are the $2$ common tangents which have the circles on opposite sides of the tangent.

In the above diagram, the $\color { red } {\text {red} }$ lines are the internal tangents.

Also see

• Two Non-Intersecting Circles have Four Common Tangents: a proof that there are $4$ such common tangents

• Definition:External Tangent

• Results about common tangents can be found here.

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): common tangent
• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): internal tangent
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): common tangent
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): internal tangent