Coin rotation, also called die axis, is the rotation of the reverse die in relation to the obverse die. Coin rotation is also sometimes specifically used to describe a 180 degree die axis, also called standard rotation or standard die axis. Coin rotation is the most common die alignment for late Roman coins. Die axis may be expressed in degrees, or hours as on a clock face, or with an arrow.
To determine the die axis of a coin, hold the coin between your index finger and thumb with the obverse facing toward you. Align the coin so your index finger is at the top of the obverse design and your thumb is at the bottom of the obverse design. Using your other hand, turn the coin between your fingers in a manner similar to a globe turning on stand with your fingers acting as the poles. Turn the coin until the reverse is facing you. Examine the reverse. If it is right-side-up, the die axis is 0 degrees or 12. If the reverse is upside down, the die axis is 180 degrees or 6. If the top of the coin reverse is to the right, at three o'clock the die axis is 90 degrees or 3. Die axis varies from 0 to 359 degrees or 12:00 to 11:59 and may be described in the same manner. If the die axis is expressed in degrees it is normally rounded to the nearest 15 degree increment. If the die axis is expressed in hours it is normally rounded to the nearest hour but sometimes half hour increments are used.
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 medalic rotation. Types struck with dies held in a hinged or other mechanical device will often have a fairly consistant standard die axis. Most modern and many ancient coins were 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.