I have finally returned to and finished a series of TessieLetter typefaces that complement the eleven Tessie typefaces (here, here, here, and here) that allow one to create tessellation patterns of birds, animals, bugs, and variety of other shapes. TessieLetters is made up of seven different fonts (each in solid and outline styles) that contain all the letters of the alphabet as well as the numbers. You can find them at fontspring.com: here, here, here, here, here, here, and here and at myfonts here.
One of the seven in the series contains only shapes that can be tessellated using a single key. Somewhat surprisingly, I was able to find ways to do the complete alphabet in this way, though some of the letters require a bit of imagination. (U, T, and G are not ideal.) A breakthrough came when I figured out a way to form the letter P. The same shape works for lower-case b, d, q and letters 6 and 9. Below is a picture of the entire one-key tessellating alphabet.
All these patterns would fit as Heesch types TTTT or TTTTTT. Many have symmetry that would allow them to fit in other Heesch types as well. Also, for several letters (such as f, h, m, n, s, and z) there are multiple shapes that work.
I am unaware of anyone else who has constructed an alphabet with this property.
In the spirit of the season:
(Cross posted at nohypetype.blogspot.com)