Plain-language explanations of the ideas behind software - compilers, garbage collection, objects, types.
The proprietary-software contract presented to a user before or during installation, often as a shrink-wrap or click-wrap agreement requiring an 'I agree' to proceed; it typically restricts copying, modification, and reverse engineering, standing in deliberate contrast to free software licenses that grant freedoms rather than withhold them.
The question of whether code under two different licenses can be legally combined into one work; copyleft licenses such as the GPL are largely one-way compatible, and conflicting terms (such as the original BSD advertising clause) can make otherwise-free licenses incompatible.
The legal instrument that grants rights to use, copy, modify, or distribute software on top of the exclusive rights copyright reserves to the author, spanning a spectrum from restrictive proprietary EULAs to free and open source licenses that grant broad freedoms.
Coloring source code by lexical and syntactic role so structure is visible at a glance, evolving from regular-expression tokenizers to full grammar-based engines like tree-sitter.