Envelopes are of two basic sizes.  First, there is the
number ten or large envelope.  Secondly, we have the number
six or small envelope.  The large envelope is commonly used
for business letters, while the small envelope is generally
used for personal letters.  However, this is not always the
case; for example, a personal letter involving an enclosure
might well demand the use of a large envelope.  And nothing
is improper about using a small envelope to send a business
letter through the mail.  Both envelopes require a specific
letter fold to assure a quick entry and exit of the letter.
     The letter's return address is keyed in the upper left
corner of the envelope.  It should be started on the second
line from the top, about three spaces from the left edge of
the envelope.  For the address, begin it at the approximate
vertical center, about a half inch or so to the left of the
horizontal center.  It is important that the letter address
always be keyed in single spacing.  If this is not done the
postal service equipment will be unable to read the address
and slow delivery.  Every state has a Zip Code abbreviation
and number.  Space twice before keying the Zip Code number.