Conjunctions are words that join phrases, clauses, sentences --see References 1. Naturally, they also join ideas. Parallel ideas are those phrases or clauses that emphasize similar ideas (see References 2); the challenge is knowing which conjunctions to use to join them. Conjunctions that join ideas are called correlatives, and it takes two to create a parallel idea partnership.
Correlatives travel in pairs
Writers use two correlative conjunctions as traveling companions because the words act as equalizers for two ideas that should be regarded equally. The most recognizable example is probably Polonius in "Hamlet": "Neither a borrower nor a lender be." (see References 3). It forms the partnership of two ideas: "never be a borrower" and "never be a lender." But those two sentences joined by an "and" are cumbersome and uninspired, so Shakespeare used the correlative partnership of "neither" and "nor"; with inverted syntax, he creates a memorable parallel idea.
Equal Outcomes Need Equal Conjunctions
In situations in which either of two outcomes can be regarded with equanimity, the correlatives to use are "whether" and "or." The following ideas are balanced: "it does not matter if you accept my proposal" and "it does not matter if you do not accept my proposal." Joining these clauses with "and" would be over-wordy and employ a double negative. The conjunctions render it: "Whether you accept my proposal or not does not matter," omitting many unnecessary words.
Equal Solutions Presented As One
If there is a question with two answers, joining the statements into one requires the team of "not only" and "but also." Suppose the sentences to join are "I love her because she is open, friendly and kind" and "I love her because she is stunningly beautiful." Joined with an "and," that's an overweight compliment, but correlatives marry the statements nicely: "I love her not only because she is stunningly beautiful, but also because she is open, friendly and kind."
Other combinations of correlatives include "both . . . and" -- she is both friendly and beautiful -- "not . . . but" -- she is not my first girlfriend, but my second -- and "either . . . or" -- I will either marry her or live alone. Correlatives are not only easy to use, but also are the best idea-joiners in the conjunction family.