Select

SelectEven(clip clip)
SelectOdd(clip clip)

SelectEven makes an output video stream using only the even-numbered frames from the input. SelectOdd is its odd counterpart.

Since frames are numbered starting from zero, by human counting conventions SelectEven actually selects the first, third, fifth, etc, frames.

SelectEvery(clip clip, int step-size, int offset1 [, int offset2, ...])

A generalization of filters like SelectEven and Pulldown. The easiest way to describe it is by example:

SelectEvery(clip, 2, 0)         # identical to SelectEven(clip)
SelectEvery(clip, 2, 1)         # identical to SelectOdd(clip)
SelectEvery(clip, 10, 3, 6, 7)  # select frames 3, 6, 7, 13, 16, 17, 23, 26, 27, ...
SelectEvery(clip, 9, 0)         # select frames 0, 9, 18, 27, ...

And how about this:

# Take a 24fps progressive input clip and apply 3:2 pulldown,
# yielding a 30fps interlaced output clip
AssumeFrameBased
SeparateFields
SelectEvery(8, 0,1, 2,3,2, 5,4, 7,6,7)
Weave

SelectRangeEvery(clip clip [, int every] [, int length] [, int offset] [, bool audio])

Selects length number of frames every n frames, starting from frame offset.

int  every = 1500
int  length = 50
int  offset = 0
bool  audio = true

By default, select 50 frames every 1500 frames, starting with frame 0.

SelectRangeEvery will normally process audio. To keep the original audio, use audio=false.

Examples:

# Selects the frames 0 to 13, 280 to 293, 560 to 573, etc.
SelectRangeEvery(clip, 280, 14)

# Selects the frames 2 to 15, 282 to 295, 562 to 575, etc.
SelectRangeEvery(clip, 280, 14, 2)