A T-shaped designer or developer is a specialist who also has a lot of capabilities in other areas. T-shaped devs get their name because the stem (or vertical line) of a T represents their expertise in one area, while the top (or horizontal line) symbolizes their related skills in a broad number of areas. T-shaped designers are great to have on your team, since they come with the benefits of both specialists and generalists. The image below highlights some of the skills a T-shaped UX designer might have. In this example, the person is a visual design specialist but also has knowledge in other areas, like coding and prototyping.
People who are both generalists (highly skilled at a broad set of valuable things — the top of the T) and also experts (among the best in their field within a narrow discipline — the vertical leg of the T). This recipe is important for success at Valve.
"An expert who is too narrow has difficulty collaborating," the handbook states, while "a generalist who doesn't go deep enough in a single area ends up on the margins, not really contributing as an individual."
Valve's new Employee Handbook