For decades, TV studios have successfully used the parity rule when negotiating deals with actors on successful, long-running series, giving all cast members in the same tier “most favored nations” terms that ensure financial parity among them and peace on the set.

Shameless star Emmy Rossum is now trying to break that rule, not accepting a salary equal to that of her male co-lead William H. Macy, who has signed on the dotted line for an eighth season. The offer had been on the table but Rossum reportedly has been holding out for a salary higher than Macy’s. Warner Bros. TV, which produces Shameless, is a studio that historically has followed the parity rule to a tee, with such series as Friends and The Big Bang Theory. Creating a precedent on Shameless appears unlikely because that would open the door for similar demands on a host of other successful series. (The next Big Bang Theory contract negotiations are just around the corner).

Hanging in the balance is the future of Shameless, one of only two series ordered by the previous Showtime regime that are still on the air, along with departing comedy Episodes. Seven seasons in, Shameless remains a ratings force and a critical darling, and the network would likely renew the series with Macy and Rossum. It is unclear whether that would be the case if only Macy is signed to come back.

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When a new ensemble TV series launches, it inevitably mixes established actors who have extensive body of work and sizable quotes with up-and-comers and newcomers whose start salary, based on experience, is lower. If the show is a hit and runs for a long time, the episodic fees usually even out as sometimes lesser known actors break out and become stars in their own right. There have been attempts by some of them to leapfrog their originally more established cast mates and get higher salaries but that has almost never worked. There was such an effort in the past on behalf of initially lesser known Jim Parsons who became the breakout star of The Big Bang Theory while his colleagues Johnny Galecki and Kaley Cuoco had come in with sitcom experience and name recognition. Parsons also has won multiple Emmys for his Big Bang role. At the end of the day, WBTV ensured most favored nations terms for all three. The original casts of both blockbuster WBTV ensemble comedies, Friends and Big Bang, reached parity and stayed there. The alternative could lead to a discord on set, something studios try to avoid.

The situation was similar on ABC/ABC Studios’ Desperate Housewives where Teri Hatcher was the best known and highest paid cast member at the beginning but the four stars ultimately achieved parity. And the adult cast of ABC/20th TV’s Emmy-winning Modern Family is very close in their salary levels (Doyen Ed O’Neill, who started off higher than anyone else by a mile, maintains larger piece of the backend).

In today’s environment where actresses have been striving for equal pay with their male counterparts, it is important to make a stand. But, thanks to the parity rule, television actually has done a better job treating male and female leads equally. Grey’s Anatomy‘s Ellen Pompeo was paid the same as male lead Patrick Dempsey before he exited the series despite his higher starting point based on his resume.

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Rossum has an argument about asking for more pay than Macy as she has had more screen presence lately and she carries many of the show’s main storylines. Still, Macy, who came into the show as an Oscar nominee, has amassed three Emmy nominations and won a SAG Award for the show.

The standoff is putting a lot of pressure on TV’s unwritten parity rule. WBTV is being put in a very tough spot. It has two options — to lose the charismatic female star of Shameless and possibly a renewal of the dramedy, along with getting some bad publicity, as the equal pay issue is being inserted into the conversation, or to break parity and make it free for all with messy future cast negotiations on other series, starting with its golden goose, The Big Bang Theory.