Prince William and Princess Kate have been in love for a long time, nearly 20 years to be exact. (It is not known exactly when they began dating, but they met as freshmen at the University of St. Andrews in the fall of 2001, became friends, roommates, and were a couple by 2002 or 2003.) Over the past 20 years, their love has undoubtedly grown in many ways: in addition to being a couple for 20 years, or nearly half of their lives since they are both 41 years old, they have been married for 12 of those years, and as they have grown and matured, so has their romance. But William has a particularly romantic way he uses to communicate his love for Kate, a body language expert told the Mirror.

"William and Kate have been performing a series of romantic gestures, tie signs, and touch rituals since the beginning of their marriage to keep their royal fans, and each other, happy, but their techniques seem to be backed by receptivity," body language expert Judy James says. 'You can tell that they are a romantic and loving couple, but their body language rituals never exclude the viewer.'

Historically their love has been quite private, but since becoming Prince and Princess of Wales in September, they have been loaded with PDA. This week alone, Kate spanked William at King Charles' second coronation in Scotland and the two kissed during a polo match.

And while public displays of affection have become more frequent, James says, they are modest and not over-the-top. 'Being modest is essential to their public role and signals that, as royalty, they are a strong couple. They maintain an even and stable emotional disposition rather than a passionate high and low soap opera."

There is no official royal protocol surrounding PDA, but as with many aspects, the pace is usually set by the monarch. Queen Elizabeth and her husband of 73 years, Prince Philip, did not PDA, and most royal families avoided PDA while the queen was monarch. (Another example: the late Majesty preferred light pink or nude manicures. Therefore, the royal ladies respected Her Majesty's preference and adopted such nail polish in most cases. Now that the Queen is dead and the current monarch is no longer a public manicurist, the rules have been relaxed, as Kate did with her red manicure on Easter Sunday this year). Since the late queen's death last September, William and Kate have relaxed the once rather strict rules surrounding public displays of affection. Says James, "When entering each other's personal space on tours and visits, William in particular seems unable to avoid breaking what often seems to be his own rule of inorganic public behavior and intensifying the romance of the moment with gestures such as a small kiss on her hair." James says." The gesture is often more for him than for her because she may not be aware of the moment, but this is a trait many couples share."

Other senses, such as smell and sight, often motivate the adoption of touch. Says James, "In other words, William smells Kate's perfume and hair, and suddenly he's driven by the desire to get closer to her, and to touch her as well."

Although she sometimes expresses affection in public, according to The Sun, in private Kate uses two nicknames for her husband and says them affectionately: "Baldy" and "Big Willie." William, on the other hand, uses the nicknames "Babkins" and "DoD." Apparently it stands for "Duchess of Dolittle. (Couples who can make fun of each other stay together.)

More recently, the couple has also been heard using more traditional pet names: at the 2016 Chelsea Flower Show, they called each other "Babe" and "Darling." "Landscape architect Charlie Alborn told Grazia U.K. that when William asked Kate about the plants, she replied, 'I don't know. She replied, "I'm not sure.