  The momentum dependence of the nematic order parameter is an important
ingredient in the microscopic description of iron-based high-temperature
superconductors. While recent reports on FeSe indicate that the nematic order
parameter changes sign between electron and hole bands, detailed knowledge is
still missing for other compounds. Combining angle-resolved photoemission
spectroscopy (ARPES) with uniaxial strain tuning, we measure the nematic band
splitting in both FeSe and BaFe$_2$As$_2$ without interference from either
twinning or magnetic order. We find that the nematic order parameter exhibits
the same momentum dependence in both compounds with a sign change between the
Brillouin center and the corner. This suggests that the same microscopic
mechanism drives the nematic order in spite of the very different phase
diagrams.
