Quick! What's the difference between these two words:
おばさん
おばあさん
Calling a middle-aged lady, 「おばあさん」 will most likely be seen as an insult.
Calling an older, grandmotherly lady, 「おばさん」 will most likely be seen as a compliment.

長音 Long Vowels
Many Japanese words have long vowels. This is called:
長音
chou on
The word, 長音, itself has a 長音 sound. (Sorry, couldn't resist.) The う u sound makes the ちょ long.
- The long vowels do not add syllables: ちょ and ちょう are both one syllable.
Listen to the difference:
ちょ
ちょう
- As a result, the vowel sound, usually, will sound the same, just held longer.
- The long vowels are about twice as long as a short vowel.
Rules for making longer sounds with Hiragana
For the most part, the following makes sense. The same vowel sound is pronounced twice as long and written with the hiragana that that sound represents. The last two, え and お, have a variation in the written form, however.
- For sounds ending in an あ a sound, add an あ:
おばさん (aunt) → おばあさん (grandmother)
おばさん
おばあさん
- For sounds ending in an い i sound, add an い after it:
おじさん (uncle) → おじいさん (grandfather)
おじさん
おじいさん
- For sounds ending in a う u sound, add an う:
くこ (Chinese wolfberry 枸杞) → くうこう (airport 空港)
くこ
くうこう
- For sounds ending in a え e sound, add either an え or い. While this can be confusing, just memorize which one as you learn vocabulary words.
おねえさん (older sister; young lady お姉さん)
へいき (calmness 平気 -or- military arms and weapons 兵器)
おねえさん
へいき
- For sounds ending in a お o sound, add either an お or an う. As with the え sound, you will just have to memorize these words. However, most of them use う.
おき (open sea; okinawa 沖) → おおきい (big 大きい; notice this has two long vowels. お for the お sound and い for the き sound)
とき (time 時) → とうき (porcelain; china 陶器)
おき
とき
おおきい
とうき
Rules for making longer sounds with Katakana
This is much easier than hiragana. Instead of adding another vowel, simply add a dash after it. ー
コーヒー coffee
アパート apartment
バター butter