This article is from the The Simpsons FAQ, by Ondre Lombard wakkojr@nospam.wcat.com and Andrew Gill superluser@mail.rit.edu with numerous contributions by others.
Sometimes referred to as the "Gruesome Twosome". Patty usually wears a
necklace with round beads and doesn't part her hair. Selma wears a
necklace with oval beads and does part her hair. Selma wears earrings
shaped like the letter "s", and Patty wears triangle-shaped earrings.
Marge differentiated them for Homer in 7F15 thusly:
Homer: "Which one's Selma, again?"
Marge: "She's the one who likes Police Academy movies and Hummel
figurines, and walking through the park on clear autumn days."
Homer: "Oh, yeah yeah yeah. But I thought she was the one that didn't
like to be ... you know ... touched."
Marge: "It's Patty who chose a life of celibacy. Selma simply had
celibacy thrust upon her."
 
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