Some people think that a shirt is a shirt is a shirt is a shirt. Not so here. The shirt here is SEHK-SH-09. It was developed over several weeks in 2016 with an unhealthy amount of effort, and is the cumulative outcome of thinking about shirts a great deal since the first shirt development here, back in 2010.
The idea with the shape of the shirt is to provide a high degree of both freedom and shape. It has slightly tapered body allied to small darts in the back, and pleats — which are horizontally aligned above those darts — up at the shoulder.
Freedom and shape also applies to the sleeves. They start with good, round, structured shoulders — which are intended to hold shape and resist the pulling or dragging that can come over time with soft shoulders — but have a dart at the elbow, to help them taper down to a cuff that grips the wrist.
The shirt is of standard length for a dress shirt, so tucks tidily into trousers. The darts in the body help in this regard. However, with its curved hem, and notches at the side-seam, it looks good and proper when worn untucked, too.
The rough heavy cotton used to develop the prototypes for the shirt has the benefit of showing starkly every flaw, every tug, every drag, imbalance, and overburdened stress-point on a pattern. It leaves nowhere for a sloppy shape to hide — if you really interrogate it, that is.