The Distributive Suffix (-ittain / -ittäin)
Add the suffix -ittain or -ittäin to a noun's plural stem to express how things are grouped, sorted, or distributed. This creates an adverb meaning "in batches of," "by," or "in groups of".
The suffix follows vowel harmony (-ittain for back vowels like a, o, u, and -ittäin for front vowels like ä, ö, y). Because it attaches to the plural stem, it always begins with -i-.
Groupings and Amounts
Use this suffix to describe how items are grouped, such as "individually" (yksittäin), "in pairs" (pareittain), or "in groups" (ryhmittäin).
It also expresses large quantities, like "by the tens" (kymmenittäin) or "by the hundreds" (sadoittain).
Progression and Sorting
This suffix can describe how an action progresses over time. The root vähä- (a little) becomes vähittäin ("gradually" or "little by little").
It also explains how things are sorted. Take the word aihe (topic):
Adding the distributive suffix gives us aiheittain (by topic).
Similarly, the noun vuoro (turn or shift) becomes vuoroittain to describe an action done in turns or shifts.