The next several months were spent playing with ideas and building prototypes on the back porch. The neighbors in our six-flat were so kind about David's loud power tools and the inordinate amounts of sawdust we created. And that's where the cool stuff happened.
A major design revision came about because of a persistent irk: The original 'popsicle' set was was limited to structures of about three feet wide, or six feet if we used long (and very heavy) pieces David nicknamed the "Three-zee". So-called because it had three notches: 
There is plenty of charm in that original set (it's on display in our shop) but one cannot ignore a persistent irk.
So, David (now officially titled Product Designer) dreamed up with a way to build the Two-zee pieces so that the ends overlap with the ends of other Two-zees. Each Two-zee has two parts, held together by magnets. This opened up untold dimensions of possibility and the number of potential Bilderhoos configurations went sky high.
Then we added a notch in the middle of every Two-zee and had to start calling them Three-zees, and at the same time we created a shorter piece which took on the name Two-zee.
If that isn't confusing enough, there are 'floor notches' in the some of the pieces that we mus tremember to refer to as 'floor cutouts' because the word notches is already taken by the One-zees, Two-zees and Three-zees. Help me, Mr Wizard!