Anything weighing more than 170gsm is generally classified as board . At this weight a second measure can be used that indicates the thickness of the sheet. This measurement is given in microns where 1 micron = 1/1000 of a millimetre. This can be confusing as different sheets may weigh the same in GSM but have different thicknesses in microns. For example a piece of balsa wood and a piece of oak may be the same thickness but will weigh very different amounts. This is due to a number of factors during the manufacture of the board. These can include the type of coating on the board, the amount of virgin fibres used or bulkiness of the raw material.