Window Cills or Sills: What's the Difference? (And Does It Matter?)

Confused about whether to say 'window cill' or 'window sill'? You're not alone. Here's the definitive guide to the spelling debate – and why it matters less than you think.

A modern window installation – but is that a cill or a sill beneath it?

The Great Spelling Debate: Cill or Sill?

If you've ever searched for window ledges online, you've probably noticed something confusing: some websites say "window cill" while others say "window sill."

So which is correct? Are they different things? Does it even matter?

Let's settle this once and for all.

The Short Answer

Both spellings are correct.

"Cill" and "sill" refer to the exact same thing – the horizontal ledge at the bottom of a window opening. There is no difference in meaning, construction, or function between them.

The only difference is regional and professional preference.

Where Each Spelling Comes From

"Sill" – The Older English Spelling

"Sill" has Germanic origins and has been used in English for centuries. It originally meant any horizontal timber at the base of a structure.

In everyday English, especially in:

United States
Canada
Australia
General British English

...you'll typically see "window sill" as the standard spelling.

"Cill" – The Construction Industry Spelling

"Cill" is predominantly used in the British and Irish construction industry. It appears in:

Building regulations and technical documents
Architectural drawings and specifications
Trade catalogues and supplier websites
Professional certifications (NSAI, BBA, etc.)

Nobody knows exactly why the construction industry adopted "cill" instead of "sill." The most likely explanation is that early technical standards simply used this spelling, and it became entrenched.

Regional Differences

Region	Common Spelling
Ireland	Cill (trade), Sill (general)
UK	Cill (trade), Sill (general)
USA	Sill (always)
Canada	Sill (always)
Australia	Sill (usually)

If you're searching for products in Ireland or the UK, you'll often get better results searching for "window cill" – because that's what suppliers call them.

If you're in North America, search for "window sill."

What About "Cills" vs "Sills" Plural?

Same rule applies:

Window cills = plural, British/Irish trade usage
Window sills = plural, general/American usage

Does the Spelling Affect What You're Buying?

Absolutely not.

Whether a product is labelled "aluminium window cill" or "aluminium window sill," you're getting the same thing:

Same material (aluminium, uPVC, stone, etc.)
Same function (weather protection, aesthetics)
Same installation (onto the window opening)

The only thing that changes is the label.

What Actually Matters When Buying

Forget the spelling – focus on these:

Material – aluminium, uPVC, stone, composite?
Dimensions – width, projection, upstand height
Finish – powder coated, anodised, painted?
Colour – RAL code for exact matching
End caps – factory fitted or site applied?

These determine quality and suitability. The spelling determines nothing.

Why We Use "Cills"

At Cills.ie, we use the "cill" spelling because:

It's the Irish construction industry standard – architects, builders, and contractors in Ireland expect to see "cill" on technical documents.
It's what our customers search for – Irish homeowners and tradespeople typically search "window cills" or "aluminium cills."
It matches building regulations – Irish building standards and NSAI documentation use "cill."
It's our name – Cills.ie. It would be a bit odd if we spelled it differently!

But we understand both spellings perfectly. If you ask for "window sills," we know exactly what you mean.

Common Search Variations

People search for window ledges in many ways:

Search Term	Monthly Searches
window sill	High
window cill	Medium-High
window sill or cill	Medium
window cills or sills	Medium
aluminium window sill	Medium
aluminium window cill	Medium
external window sill	Medium
window ledge	Medium

All of these lead to the same product. The terminology doesn't change what you need.

Technical Terms That Use "Cill"

In construction, you'll encounter these "cill" terms:

Sub-Cill

The base material (concrete, block, or timber) beneath the decorative cill. The sub-cill provides structural support; the cill provides weather protection and aesthetics.

Cill Board

In older timber window installations, the cill board is the internal window ledge – typically painted timber.

Cill Flashing

A weathering detail where the cill meets the wall, typically sealed with silicone or a metal flashing strip.

Upstand / Cill Upstand

The vertical back edge of a cill that tucks behind the window frame. Essential for weather protection.

Drip Edge / Cill Nose

The front projection of a cill with a groove underneath to prevent water tracking back to the wall.

Other Spelling Confusion

Window cills aren't the only term with variant spellings in construction:

British/Irish	American
Cill	Sill
Aluminium	Aluminum
Colour	Color
Centre	Center
Grey	Gray
Metre	Meter

If you're buying products in Ireland or the UK, use the British spellings for best search results.

Does Spelling Affect SEO?

If you run a construction website (like we do), you need to account for both spellings.

Someone searching "window sill replacement" and someone searching "window cill replacement" are looking for the same thing. A good website should rank for both.

That's why we include both spellings in our content. We want to help everyone find what they need, regardless of which version they type.

Summary: Cill or Sill?

Question	Answer
Is "cill" correct?	Yes
Is "sill" correct?	Yes
Are they different?	No – same thing
Which should I use?	Whichever you prefer
Which do Irish suppliers use?	Usually "cill"
Does spelling affect the product?	Not at all