Letter Envelopes

Mailing envelopes are a basic element of equipment in every office, secretariat and public office. They are used for the safe mailing of documents, invoices, contracts, notifications and all business and private correspondence. They are characterized by a practical construction with a closing flap placed along the longer side, which distinguishes them from document envelopes. In the Netuno.pl offer, you will find a wide selection of mailing envelopes in various formats, weights and closing systems, adapted to the needs of small companies, large enterprises and individual customers.

Mailing envelope and document envelope, the basic difference

There are two basic types of envelopes on the postal packaging market, differing in construction and purpose. Mailing envelopes have a closing flap located along the longer side, which makes it easier to quickly insert documents folded vertically. This type of envelope is the standard in everyday office correspondence, as it allows efficient packing of letters, invoices and notifications.

Document envelopes have a closing flap placed along the shorter side. This solution makes the documents inserted into the envelope more stable during transport and prevents them from shifting inside. Document envelopes are usually made from higher weight paper, which increases their resistance to mechanical damage. They are used mainly for sending larger quantities of documents, catalogs, contracts or official files.

The choice between a mailing envelope and a document envelope depends on the type of correspondence being sent. For standard letters, invoices and single documents, mailing envelopes work best. When sending thicker document files, catalogs or materials that require better protection, it is worth choosing document envelopes.

Materials used to produce mailing envelopes

Offset paper, white envelopes

Most white mailing envelopes are produced from offset paper. This is uncoated woodfree graphic paper characterized by good stiffness, high whiteness and a smooth surface. Offset paper is perfectly suitable for both black and white and color printing, so envelopes made from this material can easily be personalized with a company logo, contact details or graphic elements.

White offset envelopes are produced in weights from 75 g/m² to 120 g/m². Standard office envelopes usually have a weight of 75 to 90 g/m², which provides adequate durability while maintaining an affordable price. Premium envelopes are made from paper with a weight of 100 to 120 g/m², giving them greater stiffness, an elegant appearance and better protection of the contents.

The uncoated surface of offset paper allows free writing on the envelope with a pen or marker, as well as stamping without the risk of ink smudging. This feature makes offset envelopes a universal solution for all types of correspondence.

Kraft paper, brown envelopes

Brown envelopes are produced from kraft paper, also known as sulphate paper. The name comes from the German word Kraft, meaning strength, which perfectly reflects the main advantage of this material, exceptional mechanical durability. Kraft paper is made from long cellulose fibers subjected to special chemical processing that separates lignin from cellulose, increasing the durability of the final product.

Kraft paper envelopes are characterized by high resistance to stretching, tearing and puncturing. They are also more resistant to moisture than standard white envelopes. The natural brown color of the paper results from a production process without bleaching agents, making it an environmentally friendly solution. Kraft paper comes entirely from natural raw materials and is fully recyclable.

Brown envelopes are widely used in logistics companies, warehouses, public offices and wherever packaging durability matters. They are also often chosen by companies that focus on an eco friendly brand image.

Mailing envelope formats according to ISO 269

Mailing envelope sizes have been standardized according to the international ISO 269 standard, which defines dimensions adapted to paper formats specified in the ISO 216 standard. Thanks to this standardization, choosing the right envelopes for documents is intuitive and universal.

DL envelope, 110 × 220 mm

The DL format is by far the most popular mailing envelope size in business correspondence. The name comes from a German term meaning one third of A4 format. A standard A4 sheet folded into three equal parts fits perfectly into a DL envelope. This format most often appears in mailboxes, as it is used for sending invoices, bills, welcome letters, bank notifications, official notices and all serial correspondence.

C6 envelope, 114 × 162 mm

A C6 envelope holds A6 documents or an A4 sheet folded four times. It is one of the most popular formats in private correspondence. It works very well for sending greeting cards, invitations, postcards and photos in standard 10×15 cm and 9×13 cm sizes.

C5 envelope, 162 × 229 mm

The C5 format is intended for A5 documents or an A4 sheet folded in half. It is a universal envelope used both in companies and in private correspondence. It works perfectly for sending notifications, invitations, event tickets and larger quantities of documents.

C4 envelope, 229 × 324 mm

A C4 envelope allows A4 documents to be sent without folding. It is commonly used in offices, companies and public offices for sending important documents, reports, contracts and commercial offers. The larger format requires higher weight paper, usually at least 90 to 100 g/m².

B5 envelope, 176 × 250 mm

The B5 format is slightly larger than C5 and allows B5 documents to be inserted freely without risk of creasing. Series B envelopes are on average one centimeter larger on each side than the corresponding series C envelopes, which makes packing thicker shipments easier.

B4 envelope, 250 × 353 mm

The large B4 format allows thicker documents or items larger than A4 to be placed inside. B4 envelopes are made from paper with a weight of at least 100 g/m², which provides appropriate durability under greater load.

Mailing envelope closing systems

HK closure, self adhesive strip with protective tape

Envelopes with HK closure have a self adhesive strip protected with foil or a paper protective layer. To close the envelope, simply remove the protective strip and press the flap to the envelope body. The adhesive protected by the strip does not dry out, so HK envelopes retain their properties for more than five years when stored correctly. The closure is tight, durable and convenient to use. An additional advantage is that the envelopes can be printed easily in printers, because the protective strip prevents them from sticking to the device.

SK closure, self adhesive without tape

SK self sealing envelopes use latex based adhesive located on two sealing surfaces, on the opening flap and on the counter flap. The envelope is closed by bringing both flaps together and pressing them against each other. Manual handling is very simple and fast. The disadvantage of this solution is the limited adhesive durability, which, depending on storage conditions, is about one year. SK envelopes are an economical choice for companies that use large quantities of envelopes in a short time.

NK closure, wet sealing

Envelopes with NK closure have a layer of dry adhesive on the flap that is activated after moistening with water. This is the oldest and most traditional envelope closing system. When stored correctly, NK envelopes are practically unlimited in durability and have no expiry date. After drying, the closure is very strong and durable. NK envelopes are the standard in mass correspondence handled by automatic inserting machines.

Mailing envelopes with address window

Window envelopes are a practical solution that eliminates the need for manual addressing of shipments. The address window is a transparent film element through which the recipient address printed directly on the document inside the envelope is visible. With this solution, there is no need to print address labels or enter data by hand.

The standard address window measures 45 × 90 mm and is placed in a position that allows the address to be read by automatic postal sorting machines. Window envelopes are available in two versions, with the window on the left side or with the window on the right side. The choice of side depends on the layout of the address details on the document.

For the address to be properly visible in the window, the address details on the document should be located on the left side, about two centimeters from the left edge. The document is most often folded in a method known as a Z fold, so that the address section is on the outside and fits perfectly into the envelope window. The address should be printed in a clear, simple font, such as Arial or Times New Roman, without decorative elements that make it difficult for postal sorting machines to read the data.

Window envelopes are the standard in business and official correspondence. They work perfectly for sending invoices, bills, contracts, serial letters and all mass correspondence. The foil cutout, through which a properly folded document is visible, significantly speeds up the shipment preparation process.

Internal security print, protection of correspondence confidentiality

Internal security print is printing inside the envelope, most often in blue or gray. It is a very important element that ensures discretion and safety of the correspondence being sent. Standard white envelopes without security print may be partially transparent, especially when held against a light source. There is then a risk that confidential information contained in the document may be read by unauthorized persons without opening the envelope.

Security print effectively prevents viewing the contents of the envelope, protecting personal data, financial information, bank account numbers, PIN codes, medical documentation and other confidential content. This is especially important in business, banking and official correspondence, where data protection is crucial.

Envelopes with internal security print are the standard in professional business correspondence. Thicker paper with a weight of 90 to 100 g/m² combined with security print guarantees correspondence confidentiality and builds a positive image of the sender as an entity that cares about information security.

Mailing envelope weights

Paper weight is expressed in grams per square meter, g/m², and directly affects the thickness, stiffness and durability of the envelope. The higher the weight, the more resistant the envelope is to mechanical damage, but it is also heavier and more expensive.

Economical envelopes 75 g/m² are the cheapest solution intended for mass correspondence where price is the key factor. They are suitable for sending standard letters, advertising leaflets and correspondence that does not require special protection.

Standard envelopes 80 to 90 g/m² are an optimal combination of quality and price. They are durable enough for everyday office correspondence while maintaining an affordable price. This weight range is the most popular in DL, C6 and C5 mailing envelopes.

Premium envelopes 100 to 120 g/m² stand out with greater stiffness, an elegant appearance and better protection of the contents. Thicker paper is less transparent, which increases correspondence confidentiality even without internal security print. Higher weight envelopes are standard for larger C4 and B4 formats, where greater durability is required.

Mailing envelopes for mass and automated correspondence

Companies and institutions sending large quantities of correspondence often use automatic inserting systems. Inserting machines eliminate time consuming manual folding and packing of documents. Work that takes an hour by hand is done by a machine in a few minutes.

Envelopes intended for automatic inserting are specially designed for inserting machines. They have appropriate properties and parameters, such as high quality paper, precise dimensions and a special adhesive coating, which enable fast and precise folding, closing and addressing of envelopes by automatic devices.

Envelopes for mass correspondence are most often made with NK closure, which is activated by moistening. Inserting machines are equipped with moistening systems that automatically activate the adhesive at the right moment in the closing process. C65 format, 114 × 229 mm, and DL are the most popular sizes in automated correspondence.

Automation of the inserting process is especially useful in public offices, banks, insurance companies, telecommunications operators and wherever hundreds or thousands of letters are sent every day. By using the right envelopes and inserting machines, the mailing process can be significantly improved, saving time and human resources.

Applications of mailing envelopes

Mailing envelopes are used in practically every industry and area of everyday life. In offices, they are used for sending invoices, contracts, commercial offers, notifications and all business correspondence. Public offices use them to send administrative decisions, summons, reminders and official letters. Banks send statements, product notifications and payment cards in mailing envelopes.

In private correspondence, mailing envelopes are used for sending letters, greeting cards, invitations to family celebrations and small documents. DL and C6 formats are the most popular choices for private correspondence.

Window envelopes are especially useful for sending invoices, bills and serial letters, where the address details are already printed on the document. Envelopes without a window work better for correspondence requiring full discretion or when addressing is done on the outside of the envelope.

How to choose the right mailing envelope

When choosing a mailing envelope, it is worth following several key criteria. First, choose the envelope format according to the size of the documents being sent. Remember that the envelope should be slightly larger than the document to make packing easier and prevent creases. Second, consider paper weight depending on the nature of the correspondence. For standard letters, 80 to 90 g/m² is enough, but for important business documents it is better to choose premium envelopes of 100 g/m² and more.

The closing system should match the way the envelopes will be used. If you store envelopes for a long time before use, choose HK closure with a self adhesive strip. For quick use in large quantities, SK envelopes work well. For mass correspondence handled by machines, NK closure is the standard.

Window envelopes speed up addressing for serial correspondence, but require proper document preparation. Envelopes with internal security print are essential when sending confidential information. In the Netuno.pl store, you will find a full range of mailing envelopes in various formats, weights and variants that meet the requirements of both small companies and large institutions.

Sources

ISO 269:1985, Correspondence envelopes, Designation and sizes. ISO 216:1975, Writing paper and certain classes of printed matter, Trimmed sizes, A and B series. ISO 536:2019, Paper and board, Determination of grammage. DIN 678, Envelopes and mailing bags, Formats. DIN 476, Paper formats. Kraft process, Carl F. Dahl, Gdansk 1879. Envelope closing system specifications: HK, Haftklebeverschluss, SK, Selbstklebend, NK, Nassklebend. Poczta Polska guidelines for addressing letter mail. Technical documentation for class III offset papers, 90 to 120 g/m², and class V, 70 to 80 g/m². Netuno.pl experience in the paper and envelope industry since 2008.