Groups of witches have 13 members.
That helpful tidbit comes not from the encyclopedia, but from the Virginia Department of Motor Vehicles, which is on a mission to let state residents know just how unlucky 13 can be.
It's so unlucky that drivers should probably renew their vehicle registrations for two years instead of just one so they can avoid having a "13" sticker stuck to the back of their car for a year, the DMV says.
"It's just a publicity campaign we're running saying: 'Why don't you renew for two years so you don't have to have the unlucky number 13 on your license plate?' " said agency spokeswoman Melanie Stokes.
So take note, triskaidekaphobics -- those who fear the number 13 -- unlike traditional tag renewals, Virginia's Avoid 13 campaign comes packed with oodles of fun facts.
"Even without the phobia, many people just don't like the number 13 and would rather avoid it, if possible," the DMV notes in a recent statement. "Avoiding the number 13 can be traced back to medieval times."
The state has its own interests in helping people avoid the dreaded number. Getting people to renew for two years means less paperwork for the agency, Stokes said.
"It helps us because we're processing less transactions. Instead of having to process your vehicle registration twice, we only have to do it once," she said.
DMV employees are wearing "Avoid 13" buttons to get the word out, Stokes said. And if drivers renew for two years, they'll get a $2 discount. Three years, a $3 discount. Renewing online is worth an additional $1 discount.
Stokes said the agency has already seen people return for two-year renewals after receiving "13" stickers they consider unlucky.
"We've definitely had a few customers come in and say, 'I didn't realize I was going to be displaying 13 on my car,'" she said.
Northern Virginia residents beware: Three-year registration isn't available in Arlington, Alexandria, Fairfax County and other area jurisdictions because of emission inspection rules. Drivers in the area will likely be stuck with "14" if they don't like "13."
The Maryland Motor Vehicle Agency has no plans to remind people to avoid 13, a spokesman said.