The Body in the Library by Agatha Christie
(published 1942) ch 13
“…Why” demanded Miss Marple “was she wearing an old dress?... I think she’d wear her best dress. Girls do.”
Sir Henry interposed. “Yes, but look here Miss Marple. Suppose she was going outside to this rendezvous. Going in an open car, perhaps, or walking in some rough going. Then she’d not want to risk messing a new frock and she’d put on an old one.”
“That would be the sensible thing to do” agreed the Superintendent.
Miss Marple turned on him. She spoke with animation. “The sensible thing to do would be to change into trousers and pullover, or into tweeds. That, of course (I don’t want to be snobbish, but I’m afraid it’s unavoidable), that’s what a girl of - of our class would do. A well-bred girl” continued Miss Marple, warming to her subject, “is always very particular to wear the right clothes for the right occasion. I mean, however hot the day was, a well-bred girl would never turn up at a point-to-point in a silk flowered frock.”
“And the correct wear to meet a lover?” demanded Sir Henry.
“If she were meeting him inside the hotel or somewhere where evening dress were worn, she’d wear her best evening frock, of course – but outside she’d feel she’d look ridiculous in evening dress, and she’d wear her most attractive sports wear…Ruby, of course, wasn’t – well to put it bluntly – Ruby wasn’t a lady. She belonged to a class that wear their best clothes however unsuitable to the occasion…I think she’d have kept on the frock she was wearing – her best pink one. She’d only have changed if she’d had something newer still.”
Miss Marple, Agatha Christie’s aged supersleuth, full of good advice about what to wear if you are going to meet your murderer for a date. Very helpful. Miss Marple is investigating the murder of Ruby Keene, and the dress does turn out to be relevant. Miss M says she doesn’t want to be snobbish, but there’s no evidence (and we know how she likes evidence) that she has any objection at all to snobbery. She would, presumably, understand why Harriet Vane dressed as she did to go on a picnic with the suspect. The picture shows some girls in their best dresses. Who knows what class they are, if they are ladies or not.