In a tensile atomic force microscopy measurement, a DNA tether connects the cantilever tip and a surface. Moving the support stretches the molecule, and calibrated cantilever deflection supplies the tensile force; extension must be corrected for cantilever deflection and any linkers. At low force, extension mainly straightens an initially fluctuating coil, giving approximately linear entropic elasticity. At higher force the available conformations become increasingly aligned, so additional extension costs progressively greater force. The readable source curve then continues beyond the nominal inextensible-model contour length, which signals a failure of that simple model or of the stated parameter interpretation, rather than negative elasticity.