Given two circles with centers A and B, draw the tangents AC,
AD, BE, and BF from the center of each circle to the other.
Then, the chords MN and PQ are of equal length.
The name "eyeball theorem" may come from the fact that you can often look at a diagram of the circles and their chords and "eyeball" that the chords are the same length, without doing any actual measurements or calculations.
Static Geometry: Eyeball Theorem.
Eyeball Theorem on