This article is from the BiSexuality FAQ, by Jon Harley email@example.com with numerous contributions by others.
Dr. Alfred Kinsey created a scale, graduated between heterosexuality and
homosexuality, to rate individuals on actual experiences and psychological
reactions. The ratings are as follows:
0: Entirely heterosexual.
1: Predominantly heterosexual, only incidentally homosexual.
2: Predominantly heterosexual, but with a distinct homosexual history.
3: Equally heterosexual and homosexual.
4: Predominantly homosexual, but with a distinct heterosexual history.
5: Predominantly homosexual, only incidentally heterosexual.
6: Entirely homosexual.
Clearly anything above 0 and less than 6 can be defined as bisexual. Although
many people will say "I am Kinsey (whatever)," it should be noted that
subsequent researchers such as Klein have found it more useful to rate people
on a variety of levels, such as "Past History," "Present History," "Present
Feelings," and "Future Inclinations". Nevertheless the Kinsey scale remains a
useful tool for discussion of sexuality precisely because it is so simple.