1. Rd., St. and Sq. are short for Road, Street and square 2. #20 in the USA and No. 20 in Britain both mean Number 203. On an envelope the abbreviations c/o, Attn. and P.O.B. mean care of （转交）,attention and post office box4. You may see these in a report or textbook: e.g. or eg, i.e. or ie, etc. or etc and P.T.O.They stand for for example, that is, and so on, and please turn over5. An American firm's name may be followed by Corp. or Inc., meaning corporation6. At the end of an informal letter, you might add a P.S., in other words a postcript（附言）7. At end of a formal business letter it's common to use the abbreviations c.c. and enc. or encl., which stand for carbon copy （副本抄送）and enclosure8. But you'd only see these in very old-fashioned correspondence: ult. (last month), inst. (instant month本月) and prox. (proximo month下月)