Every collection has strengths and weaknesses. Due to fortunate circumstances or conscious, focused efforts, specific areas of a collection will be developed more than others. The oeuvre of an artist may be concentrated in a single location that offers the only opportunity to study it in depth. The works might have been transferred to an-other country - for example, the drawings of Leonardo, Guercino, and Domenichino to Windsor Castle, those of Carlo Maratta to Dusseldorf, or those of Francesco Borro-mini to Vienna - or they may be treasures of national art, such as the Turner drawings in London, Andrea del Sarto and Pontormo in Florence, or Nicolas Poussin and Dela-croix in Paris.