Mostly vs. Mainly — What's the Difference?

Mostly and Mainly, while synonymous and often interchangeable, encompass subtle distinctions that fine-tune their applicability in various contexts. “Mostly” suggests that a significant portion, but not necessarily a predominant or principal portion, adheres to a particular description or category. In contrast, “Mainly” indicates that the primary, dominant, or principal part is being referred to, introducing a nuance of precedence or dominance into the context.

Applying "Mostly" implies a broad generalization without the necessity to underscore a primary element or cause. For instance, stating “I eat vegetables mostly” implies that a large part of the diet is vegetables without emphasizing it as the key component. “Mainly”, however, introduces a focal point, hinting at a principal component or reason. So, saying “I eat mainly vegetables” could suggest that vegetables are the dominant or principal part of the diet.

In diverse contexts, "Mostly" provides a general, often quantitative, observation. For example, stating that a basket of fruit is “mostly apples” suggests a larger quantity of apples without implying that apples dominate or characterize the basket. On the other hand, “Mainly” implies a character-defining dominance; stating a group is “mainly engineers” doesn’t just quantify but implies that the character or nature of the group is notably engineering-oriented.

In weather contexts, “Mostly” might refer to the greater part of the time or area, such as “mostly sunny,” without the implication that sunshine defines the day. Alternatively, “Mainly” in a similar context might hint at a defining, prevalent feature, so saying it’s “mainly sunny” may suggest that sunshine is the dominant, defining feature of the weather, perhaps influencing planning or activities.

"Mostly" might merely quantify, as in a collection that is “mostly red marbles,” pointing to the predominance of red marbles without giving them defining status. Contrarily, “Mainly” may subtly stress the defining, or characteristic feature, so a study that is “mainly qualitative” doesn’t just quantify the type of research but implies that the qualitative nature characterizes or defines it.