一も二も無く[ichimonimonaku] unhesitatingly
一言二言[hitokotofutakoto] a word or two
一石二鳥[issekinichou] killing two birds with one stone
一二に及ばず[ichininioyobazu] right away
一姫二太郎[ichihimenitarou] It's good to have a girl first and then a boy

うり二つ[urifutatsu] as alike as two melons (two peas in a pod)
黄道十二宮[koudoujuunikyuu] twelve signs of the zodiac
二度[futatabi] again
最小二乗法[saishounijouhou] least-squares method
三十二分音符[sanjuunibuonpu] 32nd note

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

アセンブリー言葉[asenburiikotoba] assembly language
一葉[ichiyou] one leaf
右葉[uyou] right lobe
押し葉[oshiba] pressed leaves
観葉植物[kanyoushokubutsu] decorative plant

棄言葉[sutekotoba] sharp parting remark
京葉[keiyou] Tokyo and Chiba
言葉[kotoba] language
言葉づかい[kotobazukai] speech
口言葉[kuchikotoba] spoken language

葵祭[aoimatsuri] hollyhock festival
菟葵[isoginchaku] sea anemone
向日葵[himawari] sunflower (Helianthus annuus)
山葵[wasabi] wasabi (Japanese horseradish)
立ち葵[tachiaoi] hollyhock (Alcea rosea)

山葵漬[wasabizuke] pickled Japanese horseradish
花葵[hanaaoi] hollyhock
天竺葵[tenjikuaoi] geranium
銭葵[zeniaoi] common mallow (Malva sylvestris var. mauritiana)