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According to the Thomson brochure "Except for formal nights, the dress code is smart casual, which means full length trousers and an open necked shirt/polo shirt for men and equivalent attire (e.g. casual separates or dresses) for ladies."...... My husband, along with many others, usually wore smart short sleeved open necked shirt. Some men still like to wear suits or jackets but most do not. We found most people made an effort to look smart, (no jeans)....... I find the words "smart casual" do seem to have different meanings with various cruise lines..... I think Thomsons have got it right where people make an effort to look good, but not too formal (except the Captain's Cocktail party where dinner jacket or tuxedo is the norm but a suit and tie is acceptable.).
On a 7 day cruise there is normally a meet the captain formal night, usually on a cruising day. other days smart & casual or casual.
As McBlain's answer. For fellers, formal night mostly DJs but smart suit not out of place. Other nights long pants & a collared shirt are fine. ........Chris.
In my experience, the dress code on Thomson in the dining room is much smarter than on Royal Caribbean, where half the men wore jeans and t shirts. These were not to be seen at all on Thomson, which was definitely 'smart casual' rather than 'casual'! The ladies were pretty dressy every night too.