The Prince and Princess of Wales have a well-rehearsed code they use to send a signal to one another to subtly communicate when they are at public engagements and events. The secret love language was spotted during the German state visit on December 3, as the pair viewed artefacts from the Royal Collection at Windsor Castle .

At one stage, Princess Catherine , 43, was seen holding a conversation with one of the curators and talking about the items. While her husband appeared interested in their chat, he soon spotted that his uncle, the Duke of Edinburgh , was approaching.

To help his wife move on to the next set of viewing items, William, 43, placed a gentle hand on Kate's back, suggesting a subtle signal that it was time to wrap up their conversation.

Prince Harry compares R

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