These words all imply adherence to a design, an arrangement or a premeditated sequence of activities. Systematic implies an overriding concept or method of organization, and suggests a thoroughgoing principle that determines the operation, control or conduct of some process: a systematic analysis of the effects of government spending on the gross national product; a systematic exposition of the relationship between slum conditions and drug addiction. Methodical , on the other hand, need not suggest a guiding principle or a complex organization, but only a planned, often step-by-step, arrangement or process. Methodical often implies routine and painstaking effort as part of a fateful or ineluctable evolution towards some end: the methodical process of weeding out snipers by a building-by-building search of the conquered city. In some contexts the word suggests extreme detachment to the point of ruthlessness: The murderer methodically bound and gagged each victim before killing them.

Orderly has a suggestion of neatness or care, and indicates an arrangement or design that is kept within predetermined limits; the emphasis is on the manner of achievement rather than on the end sought, as is most often the case with methodical : an orderly development from a predominantly rural, agricultural economy to a modern industrial economy. Orderly thus implies the absence of violence or disruption. Regular means according to a fixed rule or standard, and suggests the lack of innovation, accident or uncertainty; it emphasizes the steadiness or continuing nature of something done according to plan: a regular programme by which underprivileged children can be given extra remedial instruction. Perhaps these words can best be distinguished by contrasting them with their opposites, as here considered. Systematic is opposed to random, methodical to haphazard, orderly to chaotic, and regular to eccentric.