Wales Millennium Centre

Eight arts organisations are resident in the building. It was designed by Jonathan Adams of the Percy Thomas Partnership in Welsh slate, steel and glass.

That opening description matters because the building is routinely described as a concert hall and is not one. A lyric theatre and a concert hall are different acoustic instruments, and the Centre eventually had to build a second, separate room to do the other job. That room is the BBC Hoddinott Hall, which opened in 2009 and seats 350.

The four performance spaces, and what each is for

The four performance spaces in the Centre descend in scale from an opera house to a room of 60 people, and each exists because the one above it could not do that particular job.

The Grace Williams Studio carries the name of the composer Grace Williams, 1906 to 1977, born in Barry a few miles down the coast, whose Sea Sketches for string orchestra and Penillion are the Welsh orchestral works most often programmed. The BBC Hoddinott Hall carries the name of Alun Hoddinott, 1929 to 2008, born in Bargoed, who wrote ten symphonies and was professor of music at Cardiff.

Why the Centre had to build a second hall

An opera house is tuned dry so that consonants carry over an orchestra playing beneath in a pit, and orchestral repertoire wants a much longer decay than that, so no single room does both jobs well.

No building has ever reconciled the two, and the Wales Millennium Centre did not try. The Donald Gordon Theatre was built correctly for opera, which is what its principal resident performs. The consequence was that the BBC National Orchestra of Wales, an orchestra founded in 1928 as the BBC Welsh regional orchestra and doing both broadcast and concert work, had nowhere in the building that suited it. The Hoddinott Hall, opened in 2009 in the second phase, is the answer: a purpose-built orchestral room with a control suite attached, small enough at 350 seats to be honest and large enough to hold a full orchestra with a choir behind it.

The eight resident organisations

The eight resident arts organisations are what distinguish the Centre from a receiving house that simply hosts touring productions. They occupy the building permanently, rehearse in it and administer themselves from it, which means the Centre is a workplace on the days when nothing is being performed. Six of the eight are named below.

• Welsh National Opera, founded in 1943, which carries its own orchestra and its own chorus and is the principal user of the Donald Gordon Theatre.
• The BBC National Orchestra of Wales, founded in 1928, based in the BBC Hoddinott Hall.
• National Dance Company Wales.
• Urdd Gobaith Cymru, the Welsh-language youth movement.
• Literature Wales.
• Ty Cerdd, the body for music in Wales.

Welsh National Opera is the unusual one in that list. Most opera companies of comparable standing are attached to a single house and rarely leave it; WNO was formed in 1943 and spent six decades as a touring company before the Centre existed, which is why the presence of a permanent home in Cardiff Bay from 2004 changed the company's shape as much as the city's.

The inscription in the windows

The inscription across the front of the building is a bilingual couplet by the poet Gwyneth Lewis, one line in Welsh above one line in English, and the letters are not applied to the slate but cut through it: each character is a window of the upper floors, so that the words read as dark shapes against the stone by day and are lit from inside at night. The two lines are not translations of one another, which is the first thing to know about them.

The Welsh line is an image of making. It turns on the furnace and on awen, the Welsh word for poetic inspiration, and it treats truth as something worked out of raw material in heat, in the way glass is. The English line is about the building and about sound: it puts singing into the stone itself and sets it against a horizon, so where the Welsh is about the craft of making, the English is about what the finished room is for. Each stands as a complete statement in its own language, which is the point of the design: neither is the caption to the other. Setting the two languages at equal size and equal height on the front of the national arts centre is a piece of language policy rendered in architecture, and it is consistent with the older institution it stands alongside, the National Eisteddfod of Wales, which has been conducted entirely in Welsh since its modern annual form began in 1861.

The materials, and the reason the building looks like that

The materials are the second half of Jonathan Adams's argument. The shell is Welsh slate laid in horizontal bands of different colours quarried from different parts of the country, wrapped round a bronze-coloured steel dome, with glass at the front carrying the inscription. The banding is meant to read as strata, an eroded cliff or a cut face of rock, which places a building of 2004 in a line of Welsh industrial and geological reference rather than in the international vocabulary of glass arts centres built at the same moment elsewhere.

Whether that reading lands is a matter of taste. What is not in doubt is that the exterior is doing a job the interior cannot: the auditoria inside are enclosed, windowless, acoustically sealed boxes, as every auditorium must be, and the only part of an opera house the public can see from the street is its wrapper. The Centre spent its architectural argument on the one surface that was free to make it.

Also in this programme

• Concert venues in Cardiff: the hallsThe concert halls of south Wales
• St David's HallThe national concert hall of Wales
• Going to a concertGoing to an orchestral concert for the first time
• The eisteddfodWhat an eisteddfod is, and how the music competition works