Proper brewing of coffee requires using the correct amount of coffee grounds, extracted to the correct degree (largely determined by the correct time), at the correct temperature. More technically, coffee brewing consists of dissolving (solvation) soluble flavors from the coffee grounds in water. Specialized vocabulary and guidelines exist to discuss this, primarily various ratios, which are used to optimally brew coffee. The key concepts are:[8] Extraction Also known as "solubles yield" Ц what percentage (by weight) of the grounds are dissolved in the water. Strength Also known as "solubles concentration", as measured by Total Dissolved Solids Ц how concentrated or watery the coffee is. Brew ratio The ratio of coffee grounds (mass, in grams or ounces) to water (volume, in liters or half-gallons): how much coffee is used for a given quantity of water. These are related as follows: Strength ? Brew ratio ? Extraction which can be analyzed as the following formula: dissolved solids/water ? grounds/water ? dissolved solids/grounds A subtler issue is which solubles are dissolved Ц this depends both on solubility of different substances at different temperatures, and changes over the course of extraction. Different substances are extracted during the first 1% of brewing time than in the period from 19% extraction to 20% extraction. This is primarily affected by temperature. External images SCAA brew hart (American)[9] SCAE brew chart (European)[10] NCA brew chart (Norwegian)[11] Brewing guidelines are summarized by Brewing Control Charts which graph these elements, and center around an "ideal" rectangle indicating the target brewing range. The yield in the horizontal (x-axis), the strength is the vertical (y-axis), and a given brewing ratio determines a radial line, since for a giving brewing ratio the strength is directly proportional to the yield. Ideal yield is widely agreed to be 20±2% (18Ц22%), while ideal strength for brewed coffee varies. American standards for "ideal strength" are generally considered[citation needed] to be between 1.25±0.10% (1.15Ц1.35%), while Norwegian standards are about 1.40±0.10% (1.30Ц1.50%). European standards fall in the middle range at 1.20Ц1.45%. These are most easily achieved with a brewing ratio of 55 g/L (55 grams of coffee per litre of water) for American standards, to 63 g/L in Norwegian standards, yielding approximately 14Ц16 grams of coffee for a standard 240 ml (8.1 US fl oz) cup. These guidelines apply regardless of brewing method, with the following exceptions: Espresso is significantly different Ц it is much stronger, and has more varied extraction Dark roast coffee tastes subjectively stronger than medium roasts. Standards are based on medium roasts[citation needed], and the equivalent strength for a dark roast requires using a lower brewing ratio.