Envelopes made from original used navigation charts. Approx. C6 (114x162mm) in size so will fit an A6 card or A4 sheet folded into quarters. 
 
I don't know the GSM of the chart paper but it is a nice weight. Used chart paper may have the odd pencil mark, number, line, or fine crease. Although I pick those in good condition to use. The envelopes may be predominately white or may have lots of colour, the set you will receive will be mixed.
 
The envelopes are handmade so maybe a mm or so difference in size. The flaps have double sided tape for you to seal the envelope.
 
If using one of these envelopes to send by post I recommend adding the postal address on a white label.
 
Available as a set of 6 or 12. They are also available as an optional extra on some of my greetings cards.