As explained in the paragraph Standard Forms, there are two important types of official charter parties, namely. The "Agreed" and the "Recommended" charter parties. The printed text of an "Agreed" charter party cannot be altered whereas the printed text of the "Recommended" charter party can be altered to adapt it to the specific requirements of the negotiating parties (e.g. the Gencon charter party).
In principle, Charter parties have two parts:
1. the printed text;
2. the additional clauses (generally typed).
The printed text contains a number of clauses with or without blank spaces which must be filled in by the contracting parties (e.g. the name of the ship, cargo to be loaded, the freight, etc.). Those printed clauses form the terms and conditions of the contract and frequently they must be completed, changed or even deleted (except in case of Agreed charter parties) to reflect the correct agreement of the parties (e.g. the laydays, the demurrage, the brokerage, etc.).
A number of additional clauses, either written or typed, which are called "riders" or "side clauses" are generally added to the filled in and adapted printed text . Those additional clauses (riders, side clauses) are generally typical of a certain traffic, and over rule the printed clauses, also sometimes called main clauses.
An "addendum" or "side letter" is frequently enclosed with the charter party, containing confidential clauses which one does not wish to divulge to certain employees of one of the contracting parties. This is quite prevalent with regard to the freight or hire which, in that case, is entered in the printed text of the charter party with the words "as agreed. For the real amount the side letter must be consulted. Port agents and other persons who are involved in the chartering, are therefore not acquainted with the freight or hire and must only know the content of the printed charter party and the additional clauses to carry out their task as intermediary to one's satisfaction..
Usually, the number of additional clauses is given at the end of the printed text. For this, standard sentences can be used in box 26, Additional clauses covering special provisions, if agreed (GENCON charter as revised 1922, 1976 and 1994, box 26) such as: Additional clauses 18-35, as per attached rider, are deemed to be fully incorporated in this charter party. (Worked out GENCON charter party as revised 1922 and 1976, box 21.)
If there are addendums or side letters, then they must be numbered, e.g. Addendum no. 1, 2, etc.
In chartering, it is customary not to start a charter party from zero but to base the negotiations on previously fixed charter parties by adapting existing clauses (main or additional clauses) to the new situation. Generally this is very laboursaving and efficient and it has the advantage that certain important elements are not overlooked.
In case of sublet, it is possible that the sub-charterer is obliged to conclude the new charter party under the same terms and conditions as the head-charter party. In that case he must use identical clauses called back-to-back clauses as in the head charter party. In case of a sublet, the first charterer (he who concluded the contract of affreightment with the owner) remains in every way responsible toward the owner and therefore, the sub-charterer does not take any responsibility towards the owner for the implementation of the charter party.