The 1960 and 1960D  small dates
The Large and Small Date varieties can be distinguished in the following ways:

1)  The tail of the 6 in the Large Date is much longer than that of the small date.
2)  The top of the 1 in the Small Date is level with the top of the 9.  In the Large Date the top of the 1 is
below the top of the 9.
3)  The 0 is perfectly round the Large Date.   In the small date the 0 is "skinnier".  

The best way to see the difference is get one of each and compare for yourself.  Only the 1960 small
date is considered rare.  The 1960 D small date is a common Lincoln cent.  

The diagnostic for the 1960 proof small date is the same as the business strikes.