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We consider capillary condensation in a deep groove of width L. The transition occurs at a pressure p(co)(L) described, for large widths, by the Kelvin equation p(sat) - p(co)(L) = 2 sigma cos theta/L, where theta is the contact angle at the side walls and sigma is the surface tension. The order of the transition is determined by the contact angle of the capped end theta(cap); it is continuous if the liquid completely wets the cap, and first-order otherwise. When the transition is first-order, corner menisci at the bottom of the capillary lead to a pronounced metastability, determined by a complementary Kelvin equation Delta p(L) = 2 sigma sin theta(cap)/L. On approaching the wetting temperature of the capillary cap, the corner menisci merge and a single meniscus unbinds from the bottom of the groove. Finite-size scaling shifts, crossover behaviour and critical singularities are determined at mean-field level and beyond. Numerical and experimental results showing the continuous nature of condensation for theta(cap) = 0 and the influence of corner menisci on adsorption isotherms are presented.