The dress code for the Royal Enclosure is very strict:
Ladies are required to dress in a manner appropriate for a formal occasion. This means that: a hat must be worn; strapless dresses are not permitted; midriffs must be covered; and trouser suits, of full length, must be of matching material and colour. Gentlemen are required to wear either black or grey morning dress, including a waistcoat, with a top hat. Overseas visitors are welcome to wear the national dress of their country or Service Dress. Please note those not complying with the dress code will be asked to leave the Royal Enclosure.
I asked the Embassy if "national dress" for the USA was Air Jordans, Baggy Jeans a FUBU sweatshirt and a backwards baseball cap, but they said it wasn't, so I rented a morning coat, trousers and a top hat. As I mentioned earlier Nancy made her dress:

Actually I think we made a pretty stylish couple, but we were downright dowdy compared to some of the attendees:

Many outfits Nancy's included (although it is difficult to see, the band on Nancy's hat is made of the same fabric as the bodice of her dress --so is my tie), had hats that were coordinated to the wearer's dress,

Bows on hats were popular

As were asymmetrical hemlines on dresses

and "Architectural" designs (does this hat remind you of the control tower at LAX?)