But it also brings the shadow effect, making it difficult for others to grow while it exercises such a powerful magnetic role.

BBC: Is London good or bad for Scotland?

It came to run what was in effect a shadow state in the north and east.

BBC: Q&A: Post-war Sri Lanka

But a shadow government would in effect be set up, an idea once spurned by both sides.

ECONOMIST: Back to work

In this tinker-toy model, you might expect the late Fall and Winter to be the time with the lowest shadow rate and in effect the easiest monetary policy.

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These go through various possible scenarios for fiscal policy, exploring whether, in effect, the shadow chancellor Ed Balls is right to say that the economy would be better off tightening more slowly - ie whether we would get more growth, overall, if we saved some of the cuts for later.

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But shadow health secretary Andy Burnham predicted the changes would have the opposite effect.

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The biological basis for this complementary color effect is not well understood, nor is the depth of shadow, which is also an illusion.

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As well as handing the letter to David Cameron, the group also met with shadow ministers including Hilary Benn, Shadow Secretary of State for Communities and Local Government, to discuss the effect of cuts on Liverpool.

BBC: Liverpool group hands Downing Street anti-cuts letter