Guam currently has the type of open primary that allows a voter to decide, in the secrecy of the voting booth, which party’s primary ballot to use.  This system is also used by several states, including Minnesota, Wisconsin, and North Dakota.  According to this story, the Guam Election Commission is considering whether to change to the type of open primary at which the voter, while at the polling place on primary election day, must ask for one particular party’s primary ballot.