It is probably more common to put the article before those two, but it can be omitted when talking about things in general under some circumstances. I'm pretty sure there is an explanation of this situation in 'Advanced Grammar in Use' - a pretty popular book all over the world. I don't have a copy of it on hand to check.
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KateLeethe relationship between elephant and mahout is the key to successful training.In your sentence "elephant" and "mahout" are being used generically so the article is optional. If the sentence were refering to a particular elephant or mahout the article would be necessary.
KateLeeWhat if I change it into “between th