Earlier this year I constructed PoultrySign, a typeface in which two sets of letters fit together by interlocking. In PoultrySign, both sets of letters were trapezoidal, one with big bottoms and the other with big tops.
Looking for other possibilities for alternating letters, I realized that convex and concave shapes will snuggle together, and from that realization came a new typeface,
Lentzers. As I worked on it, I discovered that instead of having the user switch between upper and lower-case keys, aS iN tHiS pHrAsE, I could have that process automated with an OpenType feature called calt, or contextual alternatives. The calt feature tells the word processor that when it sees two upper-case letters together, it should change the second to lower-case and when it sees two lower-case letters together, it should switch the second to upper-case. You can see the result in the picture below in which concave-shaped letters alternate with convex-shaped letters.
Lentzers comes in three weights, light, regular, and bold. It is all caps or perhaps it should be classified as unicase. If one wants to use only the convex letter or only the concave letters, one can do that by turning off the contextual alternative feature. (This assumes that the word processor supports contextual alternatives. Most new ones will.)
I searched myfonts.com for similar typefaces that alternate letter shapes in this way. I found one, but it does not use the calt feature to make the alterations automatic.
Also, I revised
PoultySign by adding the same calt feature used in Lentzers.