It is phonics:  bath, bathe, birth, birther, birthed, breath, breathe, breathes, cloth, clothe, clothes, death, dither, doth, Earth, either, faith, feather, filth, heath, heather, health, hearth, Kenneth, keepeth, killeth, leather, math, mouth, moth, neather, neither, North, Perth, pithy, Ruth, sloth, South, struth! (Aussie slang), teeth, tether, tithe, truth, tho', thou, though, threw, through, thought, thorough, trough, thanks, thank-you, three, thrice, thirty, thirtieth, three-hundred, three-hundredeth, thirst, thirsty, weather, whether, wrath worth, worthy, writh, zither. (Sorry I can't think of anymore offhand, it's a long time since I learned shorthand or studied Shakespeare - Best of Luck)

3 x 7 =  21 or Blackjack!