In "The Adventure of the Speckled Band," Sherlock Holmes' client is a lady named Helen Stoner. She had witnessed her twin sister Julia Stoner die in their house and the only clue to the cause of her sister's death were her sister's last words referring to a speckled band and the sound of a whistle. The whistle was heard by Julia every night before she had died and also by Helen, while she held her sister as she died.
The whistle, was a very important clue in the case as it was meant to call back a very poisonous snake that was used to kill Julia and could have been the reason for Helen's death too, had she not approached Sherlock Holmes for his help.
But, the sound of the whistle was attributed by those investigating the death of Julia as well as by Helen herself, when Julia had told her about it, to a group of gypsies who lived in their plantation.
It was Holmes, with his extraordinary detective skills who had managed to decipher the actual purpose behind the use of the whistle and not blame the gypsies for blowing the whistle in the middle of the night.
In "The Adventure of the Speckled Band," the first mention of the gypsies comes when Helen is explaining to Holmes that her father had no friends but these wandering gypsies, and it is during this reference to them that Helen also describes them in the negative way people often did during that time, calling them "vagabonds." From the start, then, the gypsies are singled out as unwelcome, roguish beings, drawing the reader's attention as potential perpetrators of the crime being described. Helen continues to tell Holmes and Watson about the strange things that happened to her sister leading up to her death, including a conversation where her sister asked if she had heard any whistling in the night. Helen answers, "'No, I have not. It must be those wretched gypsies in the plantation.'" Not only does this continue to push the reader to blame the gypsies but it also demonstrates how Helen herself has already decided to believe in their involvement.
One of the most important references to the gypsies appears in Helen's account to Holmes, when he asks her to explain what she thinks her sister meant by crying out, "The speckled band!"
“'Were there gypsies in the plantation at the time?”
'Yes, there are nearly always some there.'
'Ah, and what did you gather from this allusion to a band–a speckled band?'
'Sometimes I have thought that it was merely the wild talk of delirium, sometimes that it may have referred to some band of people, perhaps to these very gypsies in the plantation. I do not know whether the spotted handkerchiefs which so many of them wear over their heads might have suggested the strange adjective which she used.'”
Here is a solid example of how the gypsies have thrown everyone on the wrong track. The ambiguous nature of Julia's statement, coupled with the general suspicion of the gypsies' presence, as led Helen to interpret "band" as another name for "group," or as a reference to bands of cloth. Either way, the interpretations are centered on the involvement of gypsies.
Finally, even Holmes is thrown off the scent by the inclusion of gypsies in the story. When he takes a moment to summarize for Watson the important evidence collected so far, the gypsies are listed.
“'When you combine the ideas of whistles at night, the presence of a band of gypsies who are on intimate terms with this old doctor, the fact that we have every reason to believe that the doctor has an interest in preventing his stepdaughter’s marriage, the dying allusion to a band, and, finally, the fact that Miss Helen Stoner heard a metallic clang, which might have been caused by one of those metal bars that secured the shutters falling back into its place, I think that there is good ground to think that the mystery may be cleared along those lines.'”
Later, when the mystery has been solved and it is revealed that Helen's stepfather Dr. Roylott, and not the gypsies, killed Julia, Holmes admits his mistake to Watson.
“'I had,' said he, “come to an entirely erroneous conclusion which shows, my dear Watson, how dangerous it always is to reason from insufficient data. The presence of the gypsies, and the use of the word ‘band,’ which was used by the poor girl, no doubt to explain the appearance which she had caught a hurried glimpse of by the light of her match, were sufficient to put me upon an entirely wrong scent.'"
Even the great Sherlock Holmes was taken in by suspicions over the mere presence of the gypsies, but luckily for Helen he uses new evidence to reform his conclusion as he investigates the case.
The gypsies were put in the short story to possibly confuse the reader. Sir Arthur Conan Doyle was trying to get the reader to suspect the gypsies a.k.a. red herring.