Auxiliary , like the rest of these words, refers to something that gives additional help or support. The help given is usually similar to but less important than the main effort being made: The auxiliary units are called in only when the main force has been overtaxed. Sometimes the word can refer simply to additional or supplementary resources: an oil firm with auxiliary investments in shipbuilding and car parts. Sometimes the word seems to be used almost euphemistically to avoid any suggestion of subordination or inferiority to the main item: the ladies auxiliary committee.

Like auxiliary , ancillary may refer to something useful that may or may not be called into action. The ancillary item, however, may be dissimilar though closely related to what it supports: psychology as ancillary to the study of literature. In other instances, ancillary , more than any of the other words, may suggest a dependence of the main thing on the ancillary thing, almost as if the latter were its prerequisite, or preceded it in time: Counterpoint and harmony are certainly ancillary to serious composition.

Contributory suggests a definite cause-and-effect relationship unique among these words, indicating one of several factors that result in a particular effect. The contributory cause, however, only accompanies a major cause: Underground sabotage was a contributory factor in the German retreat before the Allied onslaught. Unlike ancillary or auxiliary items, something that is contributory exists only if actually called into action; it cannot be held in abeyance and still be contributory . Also, a contributory factor may be part of destructive as well as a constructive result: Cigarette smoking may be regarded as a contributory cause of cancer in his case.

Secondary and subsidiary both clearly state the subordination or inferiority of the added factor. Secondary relates to contributory in having a cause-and-effect meaning, but unlike contributory , can refer to both causes and effects. A contributory cause, for example, may result only in a secondary effect. Secondary may be applied in any context where comparison is made between primary values, goals, benefits and less important or gratuitous ones. In this respect, subsidiary is more specific, meaning all the attached ramifications that may exist together with a primary situation: an author’s subsidiary rights in the stage and film versions of his novel; a main store’s subsidiary branches. Subsidiary , in this sense, is most like auxiliary in suggesting a similar but less important aspect of a larger whole.