It is common practice in sea-trading, however, not to draw up a charterparty from a blank form but to base negotiating upon a previous fixture, altering main terms and additional clauses alike as required. This system is both labour-saving and expedient, at the same time providing evidence to shipowners and their brokers that certain clauses they encounter in the charterparty and perhaps find unattractive have been previously agreed by other owners. In certain cases, where chartering business is sub-let by a head charterer, the sub-charterer may be restricted to negotiating strictly on the basis of the head-charterparty, using only clauses that are identical – termed back-to-back – with the main, governing contract. Each charterparty may differ in some particular aspect, some including peculiarities not seen in others. It is the task of the sea trader to be aware of the pitfalls and advantages of major charterparty forms, and for shipbrokers to advise their principals of these when conducting chartering, so that by adept negotiation the most favourable conclusion can be reached. With some documents it is commonplace during fixing to negotiate that printed sections of text be deleted or amended in some way. These negotiations are always subject, however, to the relative strengths of the parties involved and, although one or other may be fully aware of the potential pitfalls of a certain clause, it may not be possible to alter it favourably if the other party is negotiating from a position of strength. Additionally, depending upon the particular circumstances of the voyage under negotiation, certain wordings may well have little effect whilst, for another voyage and another set of circumstances, the phraseology agreed upon may make all the difference between the success of financial failure of the venture.