It tells whoever opens it what service is below. A cover reading WATER, GAS or SEWER stops the wrong utility being opened under pressure, which is a safety matter rather than a labelling nicety.

Many authorities specify the wording, the lettering height and sometimes the cover shape or colour for each service, so the marking is part of the specification.

Replacing a marked cover with an unmarked one, or the wrong marking, is a genuine hazard and is one of the more common defects found on audit.

Scrap value. Cast iron covers have a metal price, they sit unattended in public and they can be lifted by one person.

The countermeasures are locking or bolted covers, composite materials with no scrap value, and covers cast to a non-standard profile that does not fit anything else.

Composite covers have become common for exactly this reason, and the trade-off is handling: they are lighter to lift, which is good for operatives and also good for thieves if there is nothing else stopping them.

By where the cover sits, not by what is underneath it. A cover in a footway, a car park, a carriageway and a dock apron are four different specifications for the same valve.

Under-specifying is what produces the rocking, cracked or sunken cover that everyone recognises; the cover fails long before the box does.

It is worth specifying for what the surface may become rather than what it is today, since a footway that later carries service vehicles is a common and expensive surprise.

A seating that is no longer true. Either the frame has settled unevenly, the bearing surfaces have worn, or debris has built up in the seating.

A rocking cover is more than a noise problem: every wheel that crosses it hammers the seating and accelerates the wear that caused it.

Some designs address this with a machined or tapered seating, or an elastomer insert that takes up small irregularities. On an existing cover, cleaning the seat properly is the first thing to try.

Only if it is the same pattern. Covers and frames are made as matched pairs, and the seating profile varies between manufacturers even where the nominal opening is the same.

A cover that nearly fits will rock, and a rocking cover damages the frame it does not match.

Where the original pattern cannot be identified, replacing frame and cover together is usually cheaper than the repeated visits a mismatched pair generates.

It is increasingly specified, because manual handling limits apply to the people who open these every day.

That pushes toward composite covers, smaller clear openings where the duty allows, and designs with proper lifting key locations rather than a bar and brute force.

Against that, a lighter cover in a trafficked position needs to be positively secured, or passing vehicles will move it.

The clear opening is the hole you can actually reach through once the frame is in place, and it is always smaller than the overall dimension.

It governs whether a valve key, a hand or a piece of equipment fits, so it is the dimension to compare when replacing a cover on an existing chamber.

Ordering to the outside size is a frequent error, and it produces a cover that sits correctly in the road and cannot be worked through.