After both oppnents have bid, a notrump bid shows a hand with takeout shape, but with fewer values than that required for a takeout double. A takeout double therefore is used to show a stronger hand. The sandwich notrump is usually used where (1) opponents have failed to find a fit and (2) both opponents' bids have been at the one-level. This is similar to unusual notrump, except that it shows more general shape (unusual typically requires at least 5-5 in 2 suits)