The Kinsey Scale:

First published by sex researcher Alfred Kinsey and his team in 1948, the Kinsey Scale can serve as a useful model for illustrating that bisexuality applies to a wide spectrum of attraction patterns. Each number represents a part of the sexual spectrum between "0" (exclusive heterosexuality) and "6" (exclusive homosexuality). It's important to note that the Kinsey Scale does not refer to identity, but to behavior and attraction. The middle of the Kinsey Scale (Kinsey 1-5), can be described as bisexual: a mixture of same-sex and different-sex behaviors/attractions. 

Image of the Kinsey Sale, showing on the left exclusively heterosexual and on the left exclusively homosexual, with the bottom listed from 0-6 left to right.

The Klein Grid:

In 1978, psychiatrist and sex researcher Dr. Fritz Klein developed the Klein Grid in order to better illustrate the complexity and nuance of human sexuality. Like the Kinsey Scale, the Klein Grid is not meant to "diagnose" or provide a definitive label or number on anyone's sexuality. Instead, the Klein Grid is a model intended to help people see their sexuality more holistically. Klein expanded the concept of the Kinsey Scale to include past experiences and future desires in order to highlight sexual fluidity, that is to say, the many ways a person's sexuality can evolve and change over time. He also added social and psychological dimensions in order to account for the fact that sexuality is about far more than just sexual intercourse.

The Klein Sexuality grid listing the variables from a to 3 and f to 6 to represent attractions and time periods.