As part of the collaboration, they enlisted Alonso to design two folding wooden tables: a tall, slim version and a shorter, wider model.
Each features a circular top and is supported by three legs, one of which takes the form of a thin coloured pole – available in pale blue-green or white.
The rod slots through horizontal braces that branch from the other two legs, and a hole it the tabletop, to steady and complete each design. When the poles are removed, the tables fold down flat for easy storage and transportation.
The tables are updated versions of designs that Alonso created for Japanese furniture bran Karimoku New Standard in 2012.
COS has used Hay furniture in its stores for many years, including the recently revamped flagship on London's Regent Street.
"We have always admired what Hay do and share many values," said Martin Andersson, COS head of menswear design. "Our customers have a keen interest in interior design and we felt it was the right time to offer alternative products that complimented our clothing collections."
Andersson and head of womenswear design, Karin Gustaffson, have now selected a number of items from Hay's range to display and sell in COS stores.
The objects will be available from outlets on London's High Street Kensington, Vienna's Neubaugasse and in Gothenburg from September 2015, with an edited collection available online.
"Over many years we have been very happy to work with COS on so many different levels and projects," said Hay co-founders, Mette and Rolf Hay. "At Hay we see ourselves in between fashion and architecture and in working with COS we feel like we have found a perfect home."
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COS has long aimed to align itself with the architecture and design worlds. In an interview with Dezeen, managing director Marie Honda said that both fields have been "a key influence" for the brand.
Previously, COS has worked with Snarkitecture to create a warren of textile ribbons at Milan design week 2015, and brought in Nendo to create an installation celebrating its white shirt in the same space the year before.
It is also the sponsor for the annual talks programme at London's Serpentine Gallery Pavilion – a major architecture commission – and has launched limited-edition bags and shoes to coincide with past events.