As the other educator post states, Mrs. Hale and Mrs. Peters function as the de facto jury for Mrs. Wright.
From the beginning, their husbands believe Mrs. Wright is guilty as they search for evidence at the crime scene that would definitively prove it.
The detectives’ wives accompany them to the Wright home. During the few times in which husbands and wives interact, the men are dismissive of the women’s comments and conversation. This shows the fundamental differences in perspective between men and women at the time.
As a result, when Mrs. Hale and Mrs. Peters discover the dead canary, they deliberate and reach a verdict: Mrs. Wright’s husband provoked his wife with his cruelty and abuse. The two women serve as jury and judge because they find Mrs. Wright not guilty, concealing the evidence that would identify her motive.
The title is fitting because the women understand that there is no way a jury of men would fairly weigh the issues of the case. The only way that Mrs. Wright can have a fair “trial” is in the impromptu one she receives from Mrs. Hale and Mrs. Peters—her true peers.