A.19.1- The ability is written "Flanking". [CompRules 2001/07/23 - 502.3]
A.19.2- Whenever a creature without Flanking blocks a creature with Flanking, the blocking creature gets -1/-1 until end of turn. [CompRules 2001/07/23 - 502.3b]
A.19.3- Flanking is a triggered ability. [CompRules 2001/07/23 - 502.3a]
A.19.4- Flanking is cumulative. A creature with Flanking twice gives the -1/-1 twice (a total of -2/-2 but in two separate triggered abilities). [CompRules 2001/07/23 - 502.3c]
A.19.Ruling.1- This may kill the blocker prior to damage being assigned in combat, but the attacker is still blocked. See Rule C.4.8. [D'Angelo 1998/02/03]
A.19.Ruling.2- Flanking applies to blockers which are not assigned to the Flanking creature directly. For example, creatures that become blockers due to blocking a member of a band which includes the Flanking creature are subject to the Flanking -1/-1 ability. [D'Angelo 1999/05/01]
A.19.Ruling.3- Gaining Flanking after blockers are declared will have no effect on the blockers because the time for Flanking to trigger has already passed. [D'Angelo 1999/05/01]
A.19.Ruling.4- If an attacking creature has multiple instances of the Flanking ability, even one instance of Flanking on the blocking creature will negate the effect. [D'Angelo 1998/06/15]