John & Adelle Abbot held the hands in LAYOUT #1 above. At the same moment, in a parallel universe, Don & Giselle Babbit picked up the strikingly similar hands shown in LAYOUT #2.
 
Welcome to the Twilight Zone!
 
With layout #1, John Abbot cashed one round of hearts, and then led his singleton Club. Playing John for a singleton, Adelle won and returned a club for John to ruff. The Ace of Diamonds provided the setting trick. Down one!
 
In our parallel universe, holding Layout #2, Don Babbit also cashed a round of hearts and shifted to the Eight of Clubs. Playing Don for a doubleton, Giselle held up her ace on this first round to maintain defender communications, encouraging with her Nine. Their plan to beat the contract required the defense to have a quick winner in trump, and such was the case. Don pounced on the first spade and led his remaining club to Giselle. So a club ruff provided the setting trick.
 
How did Adelle and Giselle know whether to win or duck the first round of clubs?
 
The answer is that both our pairs had agreed to play an expert convention called the Singleton Switch Lead. Their standard carding was to play Ace from Ace King on opening lead in an unbid suit, or a suit first shown by the defender on lead. And they also agreed to reverse this practice if they held a side singleton. John had led the King of Hearts at trick one, so with the club switch, Adelle knew an immediate ruff was available. Don had led his Ace of Hearts, denying a side singleton, so Giselle diagnosed Don's doubleton and knew to duck the first round of clubs.

• To summarize: In situations where you normally lead the ace from ace-king, switch your practice to leading the king first to promise a side singleton, and lead your normal honor without one. 
• A switched lead usually occurs in the defender's known long suit, so is almost never mistaken for a  holding of doubleton AK.
• Singleton Switch Leads are not for everyone! They clearly affect other A-K carding methods you may use**, such as Rusinow Leads  or Ace for attitude & King for count.
• Some partnerships have agreed to lead K from AK when 5-level contracts or above. Discuss if and how the switch will apply in such cases.
• Remember that declarer is also watching! Labeling your shift as a likely singleton may hurt rather than help the defense. Defenders must judge.
• Be sure your convention card clearly indicates that you play Singleton Switch Leads, and that you adequately explain your carding when asked!

**Switching the A and K lead in a "neutral" suit (not your proposed trump suit, or promised long suit) may be made to advertise that a doubleton AK is held. Singleton swich players must judge which defensive message is intended. 

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