It's not obvious to me why the "double consonant" symbol, っ, should be a small "tsu", and I can't seem to find anything much about it on the Internet. I've noticed cases like 実 being pronounced じつ in some contexts, yet becoming じっ in combinations like 実習. This seems more than a coincidence, yet it's not obvious to me how じつしゅう would, in pronunciation, become じっしゅう. Was that, in fact, what historically happened, or is there some other explanation?