#10 and #9 - Tinted Inside Security Envelopes

Tinted Security Envelopes 
#10 envelopes and #9 envelopes are offered here.

Size of #10 envelopes:  The closed envelope's size is 
Width x Height: 9.5" x 4.125"  - 9-1/2" x 4-1/8"

Size of #9 envelopes:  The closed envelope's size is 
Width x Height: 3.875" x 8.875"  - 3-7/8" x 8-7/8"

Flap:
Regular Envelopes: The most common commercial shape has a  triangular flap
Peel & Seal Envelopes: The flap is rectangular with the glue and liner applied in one line to make it easy to peel-off
 

Business is the Typical Application for Security Envelopes

1 - Envelopes are made out of regular paper
Paper is tipycally slightly transparent and transluscent. That  characteristic can be named as "see through".

The thinner the paper, the more transparent. The fibers used in the manufacture of the paper will also determine how much "see through" you get on a certain paper.

There is always a certain level of transparency in the lighter envelopes, especially when the paper is white . Documents that have been printed with strong dark ink inside the envelopes can become visible from the outside.

Printing a tinted layer of ink inside the envelope custs and almost eliminates that transparency. 

The colors  of ink that are usually used to print that layer are blue or black

2 - When do people use security envelopes to send mail out?

When mailing valuable documents such as checks, bills or other valuables

The information  that you find in invoices , if publicly disclosed, could infringe privacy information which is unlawful.

Financial documents are always sent in security-protected envelopes. Financial data is extremely personal and cannot be exposed.

Letters asking for delayed payments, or discussing details of transactions should be carried in security-protected envelopes as well.