In pure tone audiogram the symbol X is used to mark:

(b) Air conduction (AC) in left ear(Ref. Cummings, 6th ed., 2203)Symbol X or # is used to mark air conduction in left ear in unmasked and masked condition respectively (mnemonic; cross sign indicating (crucifixion of) Jesus Christ, (though that is made differently, we all know) Who will always be in our heart (unhidden, i.e. unmasked), and the heart being on left side.After His crucifixion on the day now called Good Friday (also called God's Friday/Black Friday), His earthly body was kept in a linen with spices in a coffin (# symbol can be considered for coffin) and finally 3 days later, on Easter Jesus Christ rose from death). So easy to remember X or # is used for showing AC in left ear in unmasked and masked conditions respectively.Air conduction in right ear is marked by symbols 0 (mnemonic; zero sign because the above happening was not at all righteous and therefore we pray to God on Good Friday for our sins to The Jesus Christ and vow to become good human beings) and A in unmasked and masked conditions respectively.Another mnemonic, for people who do not believe in God, right is right so left will be wrong (X) - simple and clear! (unmasked left AC). For masked (left AC) let's change this symbol to (#), by rearranging the four limbs of X. Now as in a maths equation, right ear being equal to left ear, left being wrong (zero marks) so right is 0 (zero), clear, nothing hidden! (unmasked)Bone conduction in right ear will be marked by symbols < or (same but opposite symbols, the opening being towards the respective hand).No response in air conduction in right ear and left ear will be marked as arrow beneath the respective symbols.