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

双節棍[nunchaku] nunchaku (two linked fighting sticks)
一双[issou] pair (esp. of folding screens)
一卵性双生児[ichiranseisouseiji] identical twins
双紙[soushi] written work (esp. a bound text, as opposed to a scroll)
自己双対[jikosoutsui] self dual

双刃[moroha] double-edged
双暗車[souansha] twin propellers
双殻類[soukakurui] bivalves
双眼[sougan] both eyes