A letter is a mail item that consists of a written notice in a closed envelope without indicated value, and every other mail item that by size and weight corresponds to the conditions provided for a letter, even if it does not have a written notice in itself. A letter must be closed in order for its contents to remain protected from possible damages in transport, and without visible marking the content may not be removed or damaged. The weight of a letter may amount up to 2 kg.
A serial letter is a letter in a closed envelope rectangular shaped which length is not less then its width multiplied by 1,4.
A letter in an envelope with a transparent part: Sizes and weight are equaled to letters in regular envelopes.
The transparent part must not be trimmed by string or framed in a colored rim and must make sure that the recipient's address can be easily read.
The sender's address on the letter in a regular envelope and an envelope with a transparent part, when it is on the front side of the envelope, must be in the upper left corner.
A letter that does not fulfill these conditions is considered a non-serial letter.