Kilayim, Chapter 4, Mishnah 4. The previous Mishnah dealt with the subject of a partition: a partition dividing between two fields permits planting right up against it even what would ordinarily be considered kilayim, so that two different species may stand directly alongside each other, as long as a partition separates them. Our Mishnah continues and clarifies additional points about the nature of the laws of a partition.
"Mechitzas hakanim" - a partition of reeds:
This refers to a partition made of upright reeds, with a gap between one reed and the next. The Mishnah rules: "im ein bein kaneh lachaveiro sheloshah tefachim kedei sheyikanes hagedi - harei zu kimechitzah" - if there is not a gap of three tefachim between one reed and its neighbor, enough for a young goat to enter, this counts as a partition. The measure of three tefachim is the gap through which a small goat can push its head in; and as long as the gap is less than three tefachim, the partition is considered a full partition. Even though there is open space between the reeds, their proximity to one another - less than three tefachim - turns the entire span of reeds into a partition, and therefore one may plant right up against it, despite those open gaps.
A fence that was breached:
"gader shenifratz ad eser amos - harei hu kepesach" - a fence that was breached up to ten amos, the breach is like a doorway. A breach of less than ten amos is considered a doorway within the fence, and the fence counts as a fence even at the place of the doorway. Therefore one may plant opposite the line where the breach is located, since it is a continuation of the fence and is nothing more than a doorway.
More than ten amos - "keneged hapirtzah asur" - opposite the breach it is forbidden. A breach larger than ten amos is considered an open space, and it is no longer part of the wall and the partition, but rather a breach in the fence, and it is forbidden to plant opposite it. However, in the area where a fence actually stands, it is still considered a partition, and one may plant opposite it on both of its sides.
"Nifretzu bo pirtzos harbeh" - many breaches were made in it:
"im ha'omed merubeh al haparutz - mutar" - if the standing portion is greater than the breached portion, it is permitted. When the total of the standing fence is greater than the total of the breached space, one may plant even opposite the place of the breach. This is on condition that no single breach by itself exceeds ten amos; and wherever a breach is larger than ten amos, it is forbidden to plant opposite it.
"im haparutz merubeh al ha'omed - keneged hapirtzah asur" - if the breached portion is greater than the standing portion, it is forbidden opposite the breach. When the total of the open space is greater than the space where the fence stands, the breaches are considered problematic and it is forbidden to plant opposite them, even if they are less than ten amos. However, in any place where there is a fence measuring at least four tefachim - and this is the minimum measure required for it to be considered a fence - one may plant opposite it. And even these four tefachim may be made up of upright posts with less than three tefachim between them.
In summary: This Mishnah teaches three rules regarding the nature of a partition: a partition of reeds whose gaps are less than three tefachim is a full partition; a breach of less than ten amos is like a doorway, and one may plant opposite it, while if it is more than ten amos it is forbidden to plant opposite it, though opposite the fence itself it is permitted; and when many breaches were made, the ratio between the standing portion and the breached portion is measured, and in any place where there is a fence measuring four tefachim, one may plant opposite it.