Tea blending is the blending different teas together to produce a final product. This occurs chiefly with black tea that is blended to make most tea bags but can also occur with such teas as Pu-erh, where leaves are blended from different regions before being compressed. The aim of blending is to create a well-balanced flavour using different origins and characters. This also allows for variations in tea leaf quality and differences from season to season to be smoothed out. The one golden rule of blending is this: Every blend must taste the same as the previous one, so a consumer will not be able to detect a difference in flavour from one purchase to the next. There are various teas which have additives and/or different processing than "pure" varieties. Tea is able to easily receive any aroma, which may cause problems in processing, transportation or storage of tea, but can be also advantageously used to prepare scented teas. Tea can be flavoured in large blending drums with perfumes, flavourants, or essential oils added. Although blending and scenting teas can add an additional dimension to tea, the process may also sometimes be used to cover and obscure the quality of sub-standard teas.