Long Vowel and Double Consonant

Long Vowel
The sounds of Hiragana with long vowels are as follows;

column	hiragana that is added to the first syllables	example
a	あ	さあ
	-aa	saa
i	い	いい
	-ii	ii
u	う	すう
	-uu	suu
e	い、え	れい、ええ
	-ee	ree、ee
o	う、お	もう、おおい
	-oo	moo、ooi

For the e column, い is added in the most cases, but in some words え is traditionally added in stead of い.
(example) おねえさん(sister)
For the o column, う is added in the most cases, but in some words お is traditionally added in stead of う.
(example) とおい(far)、おおきい(big)

Double Consonant
The sounds of Hiragana containing double consonants are as follows;

When two consonants (except for "n") are in a row, the first consonant is indicated by a little つ. It should be written in the lower half of a horizontal line. This little つ is also one syllable in length.

consonant	example
kk	けっこん、がっき
	kekkon、gakki
ss	ざっし、まっすぐ
	zasshi、massugu
tt	きって、どっち
	kitte、dotchi
pp	きっぷ、はっぱ
	kippu、happa