In almost all cases, for each codepoint in GB 2312-80, the corresponding codepoint in GB 12345-90 contains the most common traditional form of the GB 2312-80 character, so that the mapping of characters in levels 1 and 2 is trivial.
The exceptions are three instances where GB2312-80 includes (in level 2) a traditional form corresponding to a simpler character in level 1. In each of these cases, the simpler character was a rare character that was substituted for a common character with more strokes. In GB 12345-90, these pairs are interchanged, so that the more common traditional form occurs in level 1. In mapping from GB 12345-90 to GB 2312-80, both forms should be mapped to the simplified form in level 1:
1-6565 -> 0-2683 # hou4 "empress" vs. "behind" 1-6637 -> 0-5164 # yu2 "I, me" vs. "remainder" 1-8765 -> 0-3520 # yao1 "one, small" vs. me5 (question particle)
GB 12345-90 also adds a level 3, containing additional variants (103) of some level 1 characters used in the traditional orthography. Their level 1 counterparts are:
1-8801 -> 0-1658 1-8836 -> 0-2910 1-8871 -> 0-4165 1-8802 -> 0-1669 1-8837 -> 0-3001 1-8872 -> 0-4285 1-8803 -> 0-1777 1-8838 -> 0-3061 1-8873 -> 0-4341 1-8804 -> 0-1780 1-8839 -> 0-3077 1-8874 -> 0-4353 1-8805 -> 0-1823 1-8840 -> 0-3143 1-8875 -> 0-4408 1-8806 -> 0-1837 1-8841 -> 0-3168 1-8876 -> 0-4408 1-8807 -> 0-1907 1-8842 -> 0-3207 1-8877 -> 0-4408 1-8808 -> 0-1969 1-8843 -> 0-3215 1-8878 -> 0-4419 1-8809 -> 0-1983 1-8844 -> 0-3216 1-8879 -> 0-4531 1-8810 -> 0-1986 1-8845 -> 0-3259 1-8880 -> 0-4537 1-8811 -> 0-2117 1-8846 -> 0-3279 1-8881 -> 0-4582 1-8812 -> 0-2119 1-8847 -> 0-3290 1-8882 -> 0-4721 1-8813 -> 0-2177 1-8848 -> 0-3317 1-8883 -> 0-4721 1-8814 -> 0-2212 1-8849 -> 0-3343 1-8884 -> 0-4744 1-8815 -> 0-2223 1-8850 -> 0-3417 1-8885 -> 0-4782 1-8816 -> 0-2281 1-8851 -> 0-3462 1-8886 -> 0-4875 1-8817 -> 0-2302 1-8852 -> 0-3525 1-8887 -> 0-5009 1-8818 -> 0-2322 1-8853 -> 0-3541 1-8888 -> 0-5022 1-8819 -> 0-2365 1-8854 -> 0-3541 1-8889 -> 0-5184 1-8820 -> 0-2420 1-8855 -> 0-3541 1-8890 -> 0-5189 1-8821 -> 0-2441 1-8856 -> 0-3554 1-8891 -> 0-5185 1-8822 -> 0-2540 1-8857 -> 0-3570 1-8892 -> 0-5224 1-8823 -> 0-2546 1-8858 -> 0-3579 1-8893 -> 0-5238 1-8824 -> 0-2567 1-8859 -> 0-1757 1-8894 -> 0-5264 1-8825 -> 0-6756 1-8860 -> 0-3827 1-8901 -> 0-5402 1-8826 -> 0-2690 1-8861 -> 0-3830 1-8902 -> 0-5427 1-8827 -> 0-2714 1-8862 -> 0-3843 1-8903 -> 0-5427 1-8828 -> 0-2756 1-8863 -> 0-3845 1-8904 -> 0-5434 1-8829 -> 0-2767 1-8864 -> 0-3851 1-8905 -> 0-5438 1-8830 -> 0-2781 1-8865 -> 0-3907 1-8906 -> 0-5454 1-8831 -> 0-2802 1-8866 -> 0-3909 1-8907 -> 0-5476 1-8832 -> 0-2824 1-8867 -> 0-4743 1-8908 -> 0-5494 1-8833 -> 0-2850 1-8868 -> 0-3979 1-8909 -> 0-5528 1-8834 -> 0-2859 1-8869 -> 0-3990 1-8835 -> 0-2886 1-8870 -> 0-4023
Since the most common traditional counterpart occurs in the position of each GB 2312-80 character, in some cases a GB 2312-80 character has been moved to level 3, while the original position is occupied by a more complex character.