Some stellar spectral lines are quite sensitive to temperature. With current techniques, we can, for example, measure temperature modulation of a few degrees. But high temperature sensitivity can also be a detriment when it masks variations in other physical parameters of interest. In such cases, we must have a way of determining the temperature precisely so that the residual differences or variations can be seen and interpreted. The situation with stellar surface features is particularly challenging because several physical variables interact simultaneously, each impressing their signature on the spectral lines. It is frequently a good plan to compare a highly temperature sensitive line to one that is insensitive or to one that has an inverse sensitivity. A good way to find suitable lines is to compare exposures of hotter and cooler stars, as illustrated in Fig. 1. Here we see lines that hardly vary (Si I), some that show modest changes (Fe I), some that show large changes (V I), and one that shows a reverse change (Fe II). Some caution is needed because differences in surface gravity and metallicity can also produce changes in line strengths. So the general idea is to play off the differences of one line compared to another and avoid any direct dependence on absolute line strengths. Yes, this is indeed nothing more than a careful refinement of determining a spectral type.