The island is part of the post glacial boulder ridge Brunkebergsåsen stretching north to south through central Stockholm forming an elongated hill on Norrmalm while narrowing and levelling off to a low edge just north of Järntorget.
Originally called Korntorget ("Grain Square"), the square is first mentioned as Järntorget in 1489, both names being used in parallel until 1513 when iron trade had surpassed corn trade in importance.