Marginal markings are now used extensively to improve the efficiency of the post office operations.  This practice began in 1913 and was used for a short time.  They were not seen again until 1994 and are two are now being used on all pane format issues.  There are three types of such markings. 

Denominations in words (THREE, FIFTEEN, FIFTY, etc.) were added to parcel post stamps in 1913 after complaints from postal clerks that all the sheets looked alike, since they were all printed in carmine rose.  Large block letter denominations were added to eliminate this problem.  It may be collected as a margin strip of two, except for the 25¢, 75¢ and $1 issues which require a strip of 3.

Parcel post stamp imprint (Imprint XIV) on the 25¢ issue (Q9).

The Edward R. Murrow commemorative was released on January 21, 1994 with a new marginal marking - a calculation of the price of the pane.  An equation is used rather than just printing the face value of the pane.  The markings come in several different price marking formats.

 Horizontal Format.

Vertical Format.

Bar codes and USPS item numbers were added to the selvage in 1998, starting with the Breast Cancer semi-postal (B1).  They are used to track inventory and sales.  The first postal issue to show a bar code was the 33¢ Los Angeles Class issue.