When choosing a waistcoat for Ascot and in particular, the Royal Enclosure, your first consideration should be the rules! The Royal Enclosure has, of course, the most stringent criteria when it comes to dress code, but the only caveat regarding waistcoats is that they should not be of the novelty variety (nor should your tie). While ladies may wear hats to get noticed, gentlemen should take a leaf out of Beau Brummell’s sartorial book and opt for a waistcoat style that will turn only the most discerning heads. Anything worn to attract attention will inevitably be attracting the wrong attention. Subtlety and quiet assurance is the name of the game here. Let the jockeys wear the bold colours.
Given Royal Ascot is the smartest event of the British Season, we would recommend opting for a double-breasted waistcoat which is generally deemed the more traditional style. Size is important - given the straight finish at the waist, it’s critical that the waistcoat is long enough to cover all of your shirt. Even an iota of peeping white cotton from beneath the waistcoat is a sacrilege that people will no doubt enjoy pointing out to you. The other stylistic device of the DB are those sweeping lapels that help to frame the tie and subsequently the face. Adding a fob watch or chain is a very nice touch, too, and much more suited to the double-breasted style than the single-breasted.
Where the single-breasted version holds an advantage is in its ability to hold pattern. There's something about the angularity of the cut which seems better suited to bolder styles, but again, each to his own. While pattern is very much allowed in the Royal Enclosure, use discretion. Muted floral motifs or geometric embroidery can look stunning but do not make the mistake of matching your tie and pocket square with the very same fabric. That would be treading far too close to gimmick territory.
If in doubt, stick with pastel colours such as pink, cornflower blue, green and ivory, and use your tie to add contrast. You're at Ascot, not a groomsman at a wedding. A navy blue polka dot tie can be expertly employed with a pale pink waistcoat for example, drawing attention upwards to your face, which is, of course, the main aim. Any clothing that distracts attention from you is failing in its role. Similar goes with the pocket square: a silk style in a hue that is in the same ballpark as your waistcoat is the safe course of action, but if you have opted for a pastel waistcoat-tie combination, then we would suggest adding some pattern with your pocket square - a cream and navy polka dot for example.
A fine example to ape is HRH Prince Charles, who wears morning dress with aplomb. He'll very often sport an ash grey morning suit with a matching waistcoat and will coordinate his tie - either blue or lilac - with a contrast collar shirt. He breaks up the muted palette with a patterned pocket square. The last point to remember is to ensure that you are wearing black leather oxfords polished within an inch of your life.