Jacoby 2NT

[barmar]

Dunno about J2N, but in general, club players tend to explain their calls in terms of convention names rather than explaining what it means, often without knowing what the standard version of convention is, or sometimes deliberately deviating from the standard. So like "SAYC" in practice refers to any 5533 or 5542-system with a strong 1NT, and "Muiderberg" in practice refers to any 2M-opening showing a 2-suiter, it is possible that among non-experts, "J2N" just refers to any use of the 2NT response as a raise.

But for purposes of explaining the meaning of the 4♣ bid, it doesn't matter how he explained 2NT. What matters is what the pair agreed to play. So if they agreed to play Jacoby 2NT, as opposed to Stenberg 2NT, 4♣ shows a second suit and responder should explain it that way.

[richlp]

The US is a big place. What's common on the East Coast may be unheard of on the West Coast, and vice versa. Not to mention all the places in between. :)

I have lived in New York (3 areas) and California, as well as now living in Nevada. Played plenty of bridge in all places. Still haven't come across anyone in person that thinks anyone but 5-5 is standard. Was there really a time the jump showed a void?

Yes, there was such a time. There still is, especially among weaker players. I play rarely nowadays, and almost always with a pick-up partner at local clubs. Part of my 10 minute system discussion is "Jacoby 2NT?" "Yes." "What are your four-level responses?" The results are about 50% suit, 25% void, and 25% "HUH????" (Remember, pick-up partner at club)