[Barone1551]

to clarify this further. The small つ　(tsu) creates a double consonant with the letter that follows it.

for example:

あさって　Asatte　（The day after tomorrow)

If you take away the small つ

あさて　 Asate (Not sure if there is a meaning lol)

so when there is a small つ　double the consonant after. Hope this helps.