This is just a copy of part of the WGA guidelines, posted here for ease of reference.
4. "Screenplay by"
(See Section III. A.6)
Screen credit for screenplay will not be shared by more than two writers, except that in unusual cases, and solely as the result of arbitration, the names of three writers or the names of writers constituting two writing teams may be used. The limitation on the number of writers applies to all feature length photoplays except episodic pictures and revues.
a. Percentage Requirements
Any writer whose work represents a contribution of more than 33% of a screenplay shall be entitled to screenplay credit, except where the screenplay is an original screenplay. In the case of an original screenplay, any subsequent writer or writing team must contribute 50% to the final screenplay.
b. Original and Non-Original Screenplays
For purposes of determining "Screenplay by" credit only, two categories of screenplays are recognized:
(1) Original screenplays (i.e., those screenplays which are not based on source material and on which the first writer writes a screenplay without there being any other intervening literary material by another writer pertaining to the project).2 If a writer is furnished or uses research material, the screenplay is still considered an original screenplay; and
(2) Non-original screenplays (i.e., screenplays based upon source material and all other screenplays not covered in (1) above, such as sequels).
c. Additional Guidelines for the Arbiters in Determining Screenplay Credit
In each case, the arbiters read any source material and all literary material provided to them in connection with the development of the final screenplay in order to assess the contribution of each writer to the final shooting script.
The percentage contribution made by writers to screenplay obviously cannot be determined by counting lines or even the number of pages to which a writer has contributed. Arbiters must take into consideration the following elements in determining whether a writer is entitled to screenplay credit:
* dramatic construction;
* original and different scenes;
* characterization or character relationships; and
It is up to the arbiters to determine which of the above-listed elements are most important to the overall values of the final screenplay in each particular case. A writer may receive credit for a contribution to any or all of the above-listed elements. It is because of the need to understand contributions to the screenplay as a whole that professional expertise is required on the part of the arbiters. For example, there have been instances in which every line of dialogue has been changed and still the arbiters have found no significant change in the screenplay as a whole. On the other hand, there have been instances where far fewer changes in dialogue have made a significant contribution to the screenplay as a whole. In addition, a change in one portion of the script may be so significant that the entire screenplay is affected by it.
It is possible to consider the writer of a story or treatment as eligible for screenplay credit, but only in those cases where the story or treatment is written in great detail, to an extent far beyond the customary requirements for a story or treatment.
d. Selection from Source Material New!!!
As a guideline for arbiters in cases involving a non-original screenplay based upon source material, it is a fundamental principle that selection of screenplay elements from the source material is a part of the creative process of writing the screenplay. Arbiters should give weight to any writer's original and unique utilization, choice, or arrangement of source material when it is present in the final shooting script, but not the employment of basic story elements which any other writer may have also selected. (See screenplay elements - Section III. B. 4.c. See story elements - Section III.A.4.)