Optical scan machines take in paper ballots, analyze the marks on them and record the results electronically. Optical-scan systems have the advantage for verifiability-minded folk of requiring paper and, thus, a potentially auditable paper trail. They're also less expensive than DRE units. However, experiments undertaken by researcher Harri Hursti indicate that optical scan units may be vulnerable to hacker attacks if the hacker was able to get a half-hour's unmonitored access to the machines. In addition, optical-scan systems are as vulnerable as any other computer system to software-based attacks, according to Brennan Center experts.

PCOS

An optical-scan voting system (see above). Stands for precinct-counted optical scan, which differs from centrally based scan systems.

SVRS

Statewide voter-registration system, one of the requirements of HAVA. Only North Dakota is exempted from having an SVRS. North Dakota has no voter registration at all, because that state is sparsely populated enough to know when anyone is voting where they shouldn't be. (Seriously, this is their system.)