Clashed dies result after dies are hammered together
without a coin blank in place. The softer (usually reverse) die receives a partial
impression of the obverse which is transferred to coins struck from that die after the damage.
Die matched coins struck from the same clashed dies. The two
Julia Domna denarii shown above are identical in die and damage placement so both were struck with the same dies following a clash of that same die set.
Differences in these coins can be attributed to die wear, striking differences
and coin wear. Note the faint outline of the bust (upside down) and the strong
'DO' incuse at the lower left of the reverse.
Two denarii above, of the same type, demonstrate the same die before and after a clash. The upper coin of this pair was struck before the dies were clashed. The lower coin shows this same die pair after damage by clashing. In this case the dies were oriented in the upright position so the incuse is not upside down. While not nearly as clear as the previous pair, the folds of the drapery of the bust show near Venus' knees and incuse legend traces show on both right (DO at 10 o'clock) and left.
As a clashed die wears, it is possible that evidence of the clash might decrease or disappear from later coins.
The diagram below, posted on the discussion board by Congius illustrates what happens to a clashed die.
Also see Brockages and Clashed Dies.