Under the guidelines agreed between the School Curriculum and Assessment Authority and the A-level boards, examiners have been told how to decide the number of marks needed for each grade.
It follows a row earlier this year when the Oxford and Cambridge Schools Examination Board was accused of giving vastly inflated marks to A-level candidates including some from top public schools.
A scathing report by SCAA chief executive Dr Nick Tate said procedures used by OCSEB - now part of the Oxford and Cambridge Examinations and Assessment Council - were "unworthy of a reputable examination board".
The new rules are aimed at making sure all four A-level boards decide on their "grade boundaries" - the marks for the upper and lower limits of the grade - in the same way.
It says a provisional grade boundary must be set separately for each exam paper and the examiners must then compare papers above and below the grade.
The code of practice stresses that the examiners must use statistical evidence to make their judgments including the previous year's results and changes in entry patterns.
George Turnbull, director of public relations at the Associated Examining Board, welcomed the new rules. "This is a consolidation of good practice and we are pleased that everyone will be able to see what we do and how we do it. It will make doubly sure that standards across the boards are uniform."
Alan Greig, SCAA's senior manager for post-16 curriculum and examinations, said: "This has been agreed between SCAA and the boards and we all agree it will lead to a more regular and reliable procedure for A-level grading. There will be less chance of inconsistencies between boards."
The new code of practice for A-levels and AS levels is also aimed at bringing procedures used for the examinations into line with those used for GCSEs.
They are aimed at making sure grading standards are the same in every subject and from one exam board to another, and that they are consistent between different syllabuses and from year to year.
The tighter procedures will also help the process of reducing the number of exam boards and syllabuses recommended by Sir Ron Dearing.
Dr Ron McLone, chief executive of the Oxford and Cambridge Examinations and Assessment Council, said: "We welcome the way the code has made more explicit what we were already doing. We will be making sure that the code is properly applied."
A preface to the new A-level and AS-level code of practice says it is based on the sound educational principles and procedures established by the exam boards and that they have agreed to make sure it is implemented.
The wording of one of the paragraphs, it says, emphasises the need to use statistical evidence when decisions are made about grade boundaries.