24p is in most cases 23.98, or 23.976 or 24000/1001 fps.

Sorry for the weird title, but I have just received that question twice in ten minutes from 48 Hour Film Project contestants. In my last post, I said we are presenting in 1080p24, and we would prefer 24p submissions. The problem is 24.000p is likely not available on 98% of the cameras being entered into the competition (the 2% that can shoot 24.000p are the two Silicon Imaging cameras used by our team -- unless some of the Red guys are entering with a prototype--that would be fun.) The rest shoot what is commonly called 23.98, but I prefer 23.976 (it matters sometimes in AE), but it is really 24000 divided 1001 frames per second or 23.9760239760... So when I said 24p, I really mean 23.976...p -- sorry for the confusion. Even our SI-2K cameras are set to 23.976. 23.976 is the new 24p. Now if you can shoot or edit 24.000, that is fine, too; just never mix 23.976 and 24.0; you get this nasty frame ghosting due to the frame blending in most NLEs. In our more recent software, we pop-up a warning when you add 24.0 to 23.976 timeline (or vice versa.) So if you have a 24.0 camera with a 24.0 timeline, we will re-master it to 23.976 for projection.

Why this weird number : 23.976 (and its direct cousin 29.97?) It goes back to when someone thought it was a good idea to add color to television. They needed to shift the frame-rate slightly so the new "NTSC" color sub-carrier didn't freak out the existing black and white TVs. Now we are stuck with it, and it still works on your mid-century B/W set (but where is the HDMI connector?)