What the Ace is, and why versions differ
The Ace began as a plain white leather court shoe and then became a canvas for seasonal trim. The constant is the silhouette: a low-top with a rounded toe, a flat rubber sole and a leather lining. The variables are the side stripe, the embroidery and the heel detail. The best-known version carries the green-and-red Web stripe with a gold-thread bee; others use the stripe alone, a different appliqué, or a contrasting heel tab. Because the name stayed the same while the trim changed, two pairs sold as "Ace" can look quite different side by side — and because the listings come from different seasons, the fit advice attached to them differs too.
Sizing: read the centimetre column, not the label
Gucci sizes the Ace on Italian numbering, and its own chart ties each size to a foot length in centimetres. The anchors worth keeping: an IT 42 corresponds to roughly a US 8.5 and about a 27 cm foot, and an IT 43 to a US 9.5 at about 28 cm. Rather than convert a US size you already trust, measure the length of your longer foot in centimetres and read that against the centimetre column. The label advice splits between versions, so a one-to-one conversion is the least reliable path. Production runs also shift a little between seasons, which is a second reason to work from a measurement rather than a number you wore once.
The check that settles a specific pair
Lay a shoe that already fits you well flat on a ruler and note the insole length in centimetres, then ask for the same measurement on the pair you are considering. Insole length is the number that transfers cleanly between brands, because it ignores the label entirely. A US 10 wearer who normally buys an EU 44 will often find that a 43 insole already matches their foot, since the Italian last used for this shape runs long. If the pair is already in hand, measure the insole first and compare it with your own shoe before judging the fit by feel — a half-size difference is easier to see on a ruler than to feel across a room.
Describing the pair you are asking about
Four details identify an Ace unambiguously: the side stripe (Web colours, or none), the embroidery on the outer side (bee, or another motif), the heel tab colour and material, and the sole colour. Say all four when you ask, plus your insole length in centimetres. If you are matching a photograph, check whether the stripe runs the full length of the side or stops short on the smaller sizes — the trim is scaled with the shoe, so a small size can show a shorter stripe. That single detail tells you whether a photo is a small size or a large one, which matters when the same style is offered across a wide size run.