The Matrix

Had a lull at work today so had a bit of a tidy!
This could be second in a series, as I previously blipped my beloved scaler.
These are matrices (official name - Siqveland bands and that is the correct spelling, pronounced si-kweev-land).
I tell my younger patients it's like a cake tin, which holds the mixture in place until its cooked. Quite a good analogy in my humble opinion