The vertical bar on the letter T represents the depth of related skills and expertise in a single field, whereas the horizontal bar is the ability to collaborate across disciplines with experts in other areas and to apply knowledge in areas of expertise other than one's own.
[1] Tim Brown, CEO of the IDEO design consultancy, endorsed this approach to résumé assessment as a method to build interdisciplinary work teams for creative processes.
Earlier references can be found;[2] in the 1980s the term "T-shaped man" was used internally by McKinsey & Company for recruiting and developing consultants and partners, both male and female.
Other shapes have also been proposed: Γ- and Μ-shaped individuals (gamma and mu, respectively) have been described by Brittany Fiore in her ethnographic work of data science research communities to indicate people with supporting strengths in computationally- and software-intensive fields.
[3][4] Similarly, π-shaped skills (after the Greek letter pi) refer to "a broad mastery of general management skills atop a few spikes of deep functional or domain expertise".