2. #20 in the USA and No. 20 in Britain both mean Number 20

3. On an envelope the abbreviations c/o, Attn. and P.O.B. mean care of （转交）,attention and post office box

4. 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 over

5. An American firm's name may be followed by Corp. or Inc., meaning corporation

6. 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 enclosure

8. 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)