Carat weight is a way of measurement for precious stones and sometimes metals. First records of this way of measurement take their place in 15th century and was mostly used by sellers to measure the weight of gold using carob trees. Only in the 19th century one carat was accepted to be of 200 milligrams. It took a few years for that standard to be spread widely and accepted everywhere. From that time our understanding of what one carat is never changed and up to this day is commonly used for both precious stones and metals.
Each carat is also divided into 100 points. A 0.4 carat diamond can be described as a 40 pointer but when a stone is more than 1ct it’s described in decimals e.g. 1.4 carat. Sometimes diamond cutters, trying to increase the weight, may cut the girdle to be thicker, the crown taller or the pavilion deeper. It makes the diamond weight bigger thus decreasing the cut quality. The reason behind it is that there is significant price difference between a 0.98 to 0.99 and a 0.99 to a 1.00 carat diamond. The same happens in a 1.99 to 2.00ct diamond.