When yarn is coloured during the manufacturing process, it is usually dyed in batches which are treated with dye at the same time in the same vat. These batches are given unique identifying numbers known as "dye lots". Slight differences in temperature, dyeing time, and other factors can affect the shade of colour of the finished yarn, so to ensure a consistent colour in a finished garment, it is best to use yarn which comes from the same dye lot.