Square Set

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What are squared set-theoretic operators--⊑, ⊒, ⊓, ⊔-- intended to denote?

I am a second-year undergraduate mathematics student, who is studying logic, set theory, category theory, computational complexity, etc., and I have continuously come across the operators ⊑, ⊒, ⊓, ⊔, and images of the latter two on their respective "sides," always without any explanation as to their use or function.

Essentially, these appear to be block-like set theoretic operators, with squared edges.

What do these operators mean in the aforementioned mathematical disciplines? Are they in any way similar to the set operators which denote unity, disjointness, containment, etc.?

Thank you.

If you're using an old textbook, i.e. checked out a book from the 1960's from the library, or reading mathematical papers, you start to see symbols that you're not always sure what they mean. Set theoretic symbols may not have been standardized as much as they have been now, so if it makes sense in the context, I would assume that these are set theoretic symbols. In some cases there is an symbol index in the back of the book that explains the symbols.

Otherwise, with no context, I will make a guess at what the symbols mean.
Usually when you see symbols like, >= or <= (it doesn't have to be these symbols exactly), we can assume the we are dealing with a partially ordered set, (poset). When we see symbols like, v or "upside down v", we assume that we are dealing with a lattice as these denote greatest and least upper bounds. A lattice is a special type of poset. In fact, sets form a lattice under the usual set inclusion rules. I'll post links below.

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