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I've seen people use gilets to layer and often they have opted to wear them under sport coats/blazers - however, as I tend to wear my suits rather tailored/slim fit, this wasn't a feasible option for me; thus I have attempted to use the gilet as an outer layer.
I think it works quite well. I like the tail of the blazer peeking out from the bottom of the gilet and I also like the contrast it creates on the sleeves, almost colour/textile blocking.
The blazer/sports coat is made from seersucker material which is a go-to during summer months and in warmer climates but I enjoyed playing with the notion of wearing something unexpected and breaking an unwritten rule, just a bit of fun, not taking myself or fashion too seriously... which I personally feel would destroy the appeal of fashion, to create, experiment and have fun.