They are material grades stating the long-term strength of the polymer, and the difference is significant.

The number relates to the minimum required strength of the material over a design life of fifty years at a reference temperature. PE100 is a higher performance resin than PE80, so for the same wall thickness it will carry more pressure - or, put the other way, a PE100 pipe can deliver the same pressure rating with a thinner wall, which means a larger bore inside the same outside diameter and better hydraulics.

PE100 is now standard for new water and gas distribution. PE80 remains in use and is found extensively in existing networks.

The practical implication is that grade and SDR must always be quoted together. Ordering 'SDR11' without the grade does not specify a pressure rating, and PE80 SDR11 and PE100 SDR11 are different pipes with different ratings. Check both are stated on the pipe marking when it arrives.

SDR is the Standard Dimension Ratio - the pipe's outside diameter divided by its wall thickness. It is a ratio, not a dimension, and that is what makes it useful.

Because it is a ratio, a given SDR describes the same relative wall thickness at every diameter, and the resulting pressure rating is therefore constant across the size range for a given material grade. SDR11 pipe has the same pressure class whether it is a small service or a large main.

A lower SDR means a thicker wall relative to the diameter, so a higher pressure rating - SDR11 is thicker-walled and higher rated than SDR17.

Two practical consequences. The bore for a given outside diameter changes with SDR, so hydraulic calculations must use the actual internal diameter rather than the nominal size. And fusion jointing requires matching SDR as well as matching diameter, because differing wall thicknesses do not fuse correctly.

Smaller diameters come in coils; larger diameters come in straight lengths, with the changeover set by what can practicably be coiled and transported.

Coils are a major advantage on service connections and small mains, because a long run can be laid without a single joint - and every joint eliminated is a potential leak removed and a fusion operation saved. Coils also suit trenchless installation, since the pipe can be fed continuously into a mole or drill.

Larger pipe is too stiff to coil and is supplied in straight lengths, typically joined by butt fusion into strings before installation.

Coiled pipe has one characteristic to manage: it retains curvature and ovality from being coiled. It needs to be allowed to relax before jointing, and re-rounding clamps are used at fusion joints. In cold weather this takes longer, and attempting to fuse a pipe that is still oval is a common cause of poor joints.

By colour and by stripes, but the conventions differ between markets and must be checked locally.

Broadly, blue is used for potable water and yellow for natural gas in many countries, with black pipe carrying coloured stripes as an alternative identification method. Purple or violet commonly indicates reclaimed or non-potable water. Black unstriped pipe is often used for industrial, drainage and above-ground applications.

However these are conventions, not a single global standard, and there are real differences between regions - a colour meaning potable water in one market may indicate something else in another.

The pipe is also printed along its length with the manufacturer, standard, material grade, SDR, diameter, batch and date. That printing, not the colour, is the authoritative identification. Read it on delivery and confirm it matches the order, because colour alone has caused gas pipe to be laid as water and the reverse.

The pipe must carry the drinking-water approval that applies in the market where it will be installed, and these are national rather than international.

Different countries operate different regimes for materials in contact with drinking water, each with its own testing and listing process. A pipe approved in one jurisdiction is not automatically acceptable in another, and specifying 'potable approved' without naming the scheme is not a specification.

The approval covers the complete material formulation, so it applies to a specific product from a specific manufacturer, not to polyethylene generally.

The same requirement extends to everything else in contact with the water - fittings, gaskets, lubricants used during jointing, and any tapping components. It is common for an approved pipe to be jointed with an unapproved lubricant, which compromises the installation. Confirm the whole system against the relevant scheme, and retain the certificates.