Singleton Switch Leads are a special treatment that experienced partnerships can play to resolve a common and often difficult defensive problem. When partners have agreed to opening leads of the Ace from holdings of ak (unless doubleton, switch players modify this agreement by opening with a lead of the King instead - when a side suit singleton is also held.

As long as leader's partner can determine that the lead was made from the Ace and King, he can be infer that partner's shift to a different plain suit is almost certain to be from a singleton and not a doubleton.

With layout #1, opening leader cashes one round of hearts, starting with the King, and then leads his singleton club. When the King holds the first trick, leader's partner is able to diagnose the singleton, and wins to return a club for partner to ruff. The Ace of Diamonds provided the setting trick. Down one!
 
In layout #2, the defense begins with the Ace of hearts, and opening leader then shifts to the club eight. Knowing the full heart position and that partner did not make the switch lead allows 3rd hand to diagnose leader's doubleton and encourage - but duck this first club to maintain defender communications, hoping for a 3rd round club ruff. After winning the first trump lead, the defense is able to take club Ace and a club ruff to beat the contract.

So 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!

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