What does 6H.6g mean in metric thread?

Metric thread, external thread has three thread grades of 4h, 6 H and 6g, and internal thread has three thread grades of 5H, 6h and 7H. In metric threads, the basic deviation between h and h is zero. The basic deviation of G is positive, and the basic deviations of E, F and G are negative. \r\n 1 and h are common tolerance zones of internal threads, and are usually not used as surface coatings or extremely thin phosphating layers. The basic deviation of G position is used in special occasions, such as thick coating, and is rarely used in general. \r\n2 and G are usually used for 6-9um thin coating. For example, the bolt required in the product drawing is 6h, and the thread before plating is 6g tolerance zone. \r\n3。 It is best to combine threads into H/g, H/h or G/H. For refined fastener threads such as bolts and nuts, the standard recommends 6H/6g.