Kate Middleton while watching the Most Noble Order of the Garter in 2008. (AAP)
Stylish straw hats worn by the Duchess of Cambridge when she was just plain Kate Middleton are to be auctioned.
The two hats were hired, not bought, for social and ceremonial events she attended with William.
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When the Duke was invested a Knight of the Garter in June 2008, Kate watched from the sidelines with Prince Harry as William paraded through the grounds of Windsor Castle.
She wore a black and white Philip Somerville hat for the occasion and was left in a fit of giggles when she saw her boyfriend in his Garter robes and cap.
Two years later she hired a black Aurora picture hat to wear to the wedding of the Duke's friend, Harry Meade, in October 2010.
Pictures of William and Kate together sparked speculation that their nuptials were imminent and the following month they announced their engagement.
The hats, each estimated to fetch between STG1000 ($A1600) and STG1500, are being sold in June by Kerry Taylor Auctions, which sold Kate's see-through net dress for STG78,000.
She wore the outfit on the catwalk of a 2002 fashion show at St Andrews University, where she was studying with William, sparking his interest.
A spokeswoman for the auctioneers said: "With an eye for economy and practicality - hats take up masses of storage space - she preferred to hire a hat for the day rather than purchase it outright.
"She was sometimes accompanied by her mother, Carole, who helped her make the right selection.
"The lots are being sold with hire receipts and copies of diary entries showing Kate's appointments at the shop."
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