Is it rigid or RIDGID?

Both rigid and RIDGID are correct spellings but refer to different things. Rigid with lowercase is common English adjective meaning stiff, inflexible, unyielding, and not bending, used to describe objects, rules, or behavior, as in rigid board or rigid rules. RIDGID in all caps is brand name for well known tool company that manufactures plumbing tools, power tools, and equipment, recognizable by orange color and trademark RIDGID. So if you mean adjective describing stiffness, it is rigid with lowercase. If you mean brand of tools, such as RIDGID pipe wrench or RIDGID shop vac, it is RIDGID in all caps as trademark. Confusion arises because brand name is same word as adjective but stylized in caps. So in general writing, use rigid for adjective, and use RIDGID only when referring to tool brand. For example, rigid insulation is adjective, RIDGID drill is brand. So both exist, meaning depends on whether you refer to generic adjective or specific tool brand.