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When you make a sacrifice for a future gain, you must feel sure that the gain is there. Where the sacrifice is a material one and the prospective gain positional, judgment is often sufficient; but where the sacrifice is positional and the prospective gain is material (especially if it is only a pawn), it is usually necessary to consider how the position may stand after the material is won. Cecil Purdy, Fine Art Of Chess Annotation And Other Thoughts - Volume Two