One-dimensional repeating patterns
There is always more than one possible unit of repeat. There are usually one or two obvious possibilities but there are often many others that are less obvious. The only requirement is that the whole object (apart possibly from its edges) can be made by repeating the unit.
The fence is a one-dimensional repeating pattern.
Its basic structure is as follows:
Two possible units of repeat are outlined above, one obvious and one not so obvious.
In both cases, extra smaller pieces have to be added at both ends in order to make the complete fence.
The track of footprints shown was made by one person's right and left feet. A possible one-dimensional unit of repeat is one footprint plus the space before the next print made by the same foot.
The ladder shown is also a one-dimensional repeating pattern, made by repeating the rungs at a fixed spacing. Extra pieces are needed at the ends in order to complete the ladder.