Die alignment, also called die axis or coin rotation, describes how the obverse and reverse dies were aligned to each other when the coin was struck. Die alignment is most often expressed in degrees, but may also be expressed as a clock face hour. Some publications use arrows to describe die alignment.
To determine die alignment, hold the coin by the edges between your thumb and index finger with the obverse facing you. Use your other hand to turn the coin until the obverse, still facing you, is perfectly right side up, still held between your thumb and index finger. Then, with your other hand push against the left side of the coin, so it rotates between your fingers like a revolving door (or a globe on its axis) until the reverse is facing you. The location of the very top of the reverse after spinning it between your fingers indicates the die rotation. The chart and clock face below shows the die rotation range from 0 to 359 degrees or from 1:00 to 12:00. If the reverse is right-side-up, the die alignment is 0 degrees or 12:00 (also called medallic rotation). If the reverse is upside down, the die alignment is 180 degrees or 6:00 (also called coin rotation). Normally die alignment is expressed as the nearest 15 degree increment (0, 15, 30, 45, 60...) or the nearest hour, but there is no firm rule. A coin with an axis between 340 and 20 degrees is likely to be described as having a 0 degree die axis, and a coin with a die axis between 160 and 200 is likely to be described as having a die axis of 180 degrees.
Most Roman coin types have a consistent die axis of either 0 or 180 degrees. Athenian tetradrachms almost always have a die rotation of 270 degrees. Some types do not show any consistency or pattern in die alignment. These issues can be described as struck with "loose dies." If a type has a consistent die alignment, deviation may indicate a forgery. Die alignment is, however, really only useful in detecting very old forgeries. Today, most forgers will know and use the correct die axis.
A die axis of 180 degrees or 6 o 'clock is called standard die axis or standard rotation. A die axis of 0 degrees or 12 o 'clock is called medallic die axis or medallic rotation. Types struck with hinged dies (dies held in a hinged or other mechanical device) will usually have a fairly consistent standard die axis. Most modern and many ancient coins were consistently struck with a standard or medallic die axis. For ancient coins variation of up to 10% from the normal standard axis is typical and often not noted in descriptions. Some coins were struck with hand held or "loose dies" and do not have a normal or standard die axis for the type.
An incorrect die axis for the type may indicate a forgery, though most modern fakes are produced with the correct die axis for the type. When a new hoard of coins appears with a consistent die axis, but is of a type struck with lose dies (without a normal set die axis), the hoard may be fake. The opposite is also true.