[Antrax]

In my partnership we currently play that after a 1♣ opening and 1♦ overcall, double by responder shows at least one four-card major and bidding a major shows 5 or more. I get the impression this is a bad idea. Can anyone explain why, and what double should be? (4-4 majors?)

[mich-b]

The popular style is for double to show 4-4 majors (rarely 5-4 with a weak hand?) and bid 1M with 4+ cards.
The reasoning is that if you double showing 4+ in either major , and the opps bid 2♦ or even worse 3♦ it may be difficult for your side to discover your fit. Having said that I played your style (1M=5) for several years , and while the unspecific double is a weakness, knowing at once that 1M is 5 cards is an advantage. I don't think your style is unplayable.
Also , some modern players who use transfer responses to 1♣ , try to still keep them after 1♦ overcall and use : Dbl=4+♥ , 1♥ = 4+♠, etc...

[TWO4BRIDGE]

The following is not standard, but has solved some problems for the folks who use it:

1C - ( 1D ) - ??
................. DBL = promises 4 cards ♠ but says "nada about ♥"
................. 1H = 4+ cards ♥
................. 1S = 5+ cards ♠

[Antrax]

Why prefer spades to hearts here?

[TWO4BRIDGE]

Antrax, on 2011-October-10, 08:11, said:

Why prefer spades to hearts here?

Additional quote from the expert who uses this treatment :
EDIT: "The Neg-DBL, promises 4 cards ♠ but not necessarily ♥......or may have 4-4 in the Majors.
If opener has hearts but not spades, he bids hearts. With both, he bids spades. This info comes in handy if the bidding gets into the stratosphere."

This post has been edited by TWO4BRIDGE: 2011-October-10, 08:33

[gwnn]

I seem to recall some structure with
x =44 in the majors but spades are exactly 4.
1♥=4+ hearts, less than 4 spades
1♠=4+ spades, (and some inference about hearts).

But maybe hearts and spades (and/or the inferences) were reversed.

Sorry about this unhelpful post. It happened in the last minute, when I realised that mich-b has already said everything I wanted to say about your initial question. I don't think "your" style in unplayable either, but the standard way is what I would prefer, and there are even better methods out there (none of which I play).