7in refers to the physical size of the record only (you can measure it to check) while 45 is one of the speeds that records play at.
Most 7inch records contain one song on each side and play at 45 (that's the number of times it spins around, per minute). EPs are also seven inch but usually contain two songs on each side.
To further confuse things (!) you can also get records that play at 45 which are 12inch - the same size as an LP. These often contain longer versions of the song on the 7inch issue.
A 12inch LP will usually play at 33 1/3 revolutions per minute and will contain a half a dozen songs on each side.
You will see dealers listing records as either 7in or 45 - if it says 45, you my need to check to see if they're 7in or 12in - if your sister's records measure 7 x 7 inches and have one song on each side you can be pretty sure that they're almost certain to all be 45s.
(You can also get records that are 10inch and records that play at 78rpm and even 16rpm)
as Glenn says a 7" is the size, they normally play at 45rpm but occasionly play at 33 when theres more music squashed onto them
45s generally is a slang term for 7"s too, hence the lyrics in cornershops brimful of asha
Mine's on the forty-five
Mohammed Ruffi (forty-five)
Lata Mangeshka (forty-five)
solid state radio (forty-five)
ferguson mono (forty-five)
Mon public (forty-five)
Jacques Dutronc and the Bolan Boogies
the Heavy Hitters and the chi chi music
All Indian radio (forty-five)
two in ones (forty-five)
ovvo records (forty-five)
trojan records (forty-five)
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