Because polyethylene is flexible, and compressing it from outside without support simply squashes it.

The fitting works by squeezing a seal and a gripping ring onto the outside of the pipe. A rigid pipe resists that and the seal compresses; PE deforms, the wall moves inward, the bore goes oval, and the sealing force is lost. The joint either leaks immediately or relaxes and leaks later as the polymer creeps.

The support liner - a metal or plastic tube inserted into the pipe bore - takes the compressive load from inside so the wall cannot collapse.

Liners are sized to the wall thickness as well as the diameter, which is the part that gets missed. Pipe of the same outside diameter but a different SDR has a different bore and needs a different liner. Fitting the wrong liner either will not enter the pipe or leaves the wall unsupported. Check both diameter and SDR when ordering.

Appropriate for service connections, small diameters, temporary work and demountable joints; not the default for buried mains.

They are the normal method for connecting a water service to a property, for meter and stop tap installations, for irrigation, and for repairs where fusion equipment is not justified. Being demountable, they suit any position where the joint may need to be broken later.

They are less appropriate on large-diameter mains, on high-pressure duty, and on long buried pipelines where fusion gives a joint as strong as the pipe with nothing to loosen. They are bulkier, which matters in a congested trench, and they generally carry a lower pressure rating than a fused joint in the same pipe.

For gas, compression fittings must be specifically approved for gas service - a water fitting is not acceptable, and the requirements differ by market.

Cut square, mark the insertion depth, fit the liner fully, insert to the mark, and tighten the nut properly.

Cut the pipe square with a proper pipe cutter, not a saw, and deburr it. Check the end is round; coiled pipe often is not, and a re-rounding tool or simply cutting back to a rounder section may be needed. Push the support liner fully home so its shoulder seats against the pipe end.

Mark the insertion depth on the pipe from the fitting body before assembly. That mark is the only way to confirm afterwards that the pipe went fully in - once the nut is on, you cannot see.

Tighten the nut to the manufacturer's figure. Many designs are tightened by hand then a defined amount with a spanner. Overtightening distorts the gripping ring and can crack a plastic nut; undertightening leaves the ring not properly engaged.

Yes - transition fittings are one of their main uses, and often the simplest way to change material.

Ranges include couplers with a PE compression end and a male or female threaded end for connecting to metallic pipework, valves and meters; flanged adaptors for larger connections; and fittings sized to join PE to copper, MDPE to imperial pipe, and similar combinations found on older services.

This matters because PE cannot be fused to anything but PE, so every material change needs a mechanical connection of some kind.

Two cautions. Check the fitting is rated for both materials, not just the PE side - a compression end designed for PE will not necessarily grip copper correctly. And on potable water, confirm the fitting and its seals carry the local drinking water approval, since transition fittings are frequently sourced separately from the pipe and the approval is easy to overlook.

Yes, and they are used underground constantly on service connections - but with more care than an above-ground joint needs.

The fitting must be rated for burial and its materials chosen for the ground conditions. Metallic bodies in aggressive or made ground should be protected, and many are supplied with a protective coating or are specified in a corrosion-resistant material for that reason.

The joint should be surrounded with clean fine bedding rather than backfilled with sharp stone, and it must not be left carrying load from the pipe either side - the pipe should be supported so the fitting is not acting as a bracket.

Record the position. A buried mechanical joint is the most likely point of future leakage on a service run, and the difference between a marked position and an unmarked one is hours of excavation later. On critical connections, fusion remains preferable precisely because there is nothing to find.