Shape notes where introduced into America in 1801 as a means to teach singers to quickly learn songs without their having to master the more complex details of music contained in the staff and key signatures. The system was primarily taught through "singing schools," and these musical gathering served as both an educational and social event for local communities.
In shape note music, every note is assigned a specific shape. There are only seven, and each shape corresponds to one of the pitches of a major (or minor) scale. For example, in a major key, the triangle is the first scale degree, the diamond is the third scale degree, and the circle is the fifth scale degree, etc. Once a singer learned to "read" the seven shapes, they could easily sight read any number of songs. It was no-doubt a tremendous asset to help congregations to learn to sing parts as well as new music. In my opinion, it was ingenious, and I'm sorry it has fallen out of favor.
Most musical traditions no longer use shape notes in their choral music, but a few months ago I remembered a sight singing method I was taught in college that closely resembled this technique but utilized standard "round note" notation. I've been experimenting with this method with my various choirs, and I think it is another valuable tool to have in one's musical toolbox. Just like shape note singing, in it's simplest format, this system teaches singers to learn to sight sing without having to first learn all the intricacies of the music staff. I'm now using it with my kids to teach new songs by rote, and I'm using it with my youth ensembles to teach them to sight sing. It is called Numerical Sight Singing. I'll post more details later about the specifics of how I'm employing this system. It is a lot simplier than it sounds. I wish I had started using it years ago.