Tm is shorthand for melting temperature. The melting temperature of a nucleic acid duplex is the temperature at which half of the strands are in duplex form, and the other half are single stranded. This is determined by taking advantage of a spectral phenomenon called hypochromicity, which is the reduction in UV absorbance that is observed when nucleic acid bases form a duplex. The act of hydrogen pairing with the opposing strand and the change in electron patterns results in the observed reduction in the spectral reading at 260 nm. By cooling a sample containing complimentary strands in a cuvette until the strands are fully hybridized (absorbance is stable), then heating the sample slowly and recording the absorbance during the process, one obtains a sigmoidal shaped plot when temperature is plotted against absorbance. The transition temperature at the inflection point is the Tm.