Colors in the NCS are defined by three values, expressed in percentages, specifying the degree of blackness ("s" , = relative visual similarity to the black elementary color), chromaticness ("c", = relative visual similarity to the "strongest", most saturated, color in that hue triangle), and hue ("Φ", = relative similarity to one or two of the chromatic elementary colors red, yellow, green and blue, expressed in at most two percentages). This means that a color can be expressed as either Y (yellow), YR (yellow with a red component), R (Red), RB (red with a blue component), B (blue), etc. No hue is considered to have visual similarity to both hues of an opponent pair; i.e. there is no "redgreen" or "yellowblue". The blackness and the chromaticness together add up to less than or equal to 100%. The remainder from 100%, if any, gives the amount of whiteness ("w"). Achromatic colors, i.e. colors that lack chromatic contents (ranging from black, to grey and finally white), have their hue component replaced with a capital "N", for example "NCS S 9000-N" (a more or less complete black). NCS color notations are sometimes prepended by a capital "S", which denotes that the current version of the NCS color standard was used to specify the color.