Necklace length 40+2.5 refers to the one-dimensional space measurement of chain jewelry made of gold, silver and jewelry. The length hanging around the neck is 40 cm plus 2.5 cm. Necklace is a chain jewelry made of gold, silver and jewelry. Its length is a measure of one-dimensional space (point-to-point distance), and 40+2.5 represents the total length and extended length of the necklace (necklace length+extended chain length, length unit is cm). Therefore, linking means one-dimensional spatial measurement of chain jewelry made of gold, silver, jewelry and so on. The length hanging around the neck is 40 cm plus 2.5 cm.