下位互換性[kaigokansei] downward compatibility
互い[tagai] mutual
互いに[tagaini] mutually
互い違い[tagaichigai] alternate
互い違いに[tagaichigaini] alternate

互に[katamini] mutually
互角[gokaku] equality
互換[gokan] transposition
互換性[gokansei] compatibility
互恵[gokei] reciprocity