It is crucial that you write your harmonizations within the natural range of these voices because otherwise you may end up writing too high or too low. It helps here to imagine your harmonization being sung by a SATB choir.
If you write too high for any voice to sing, then they have to really struggle to perform that line. The result would be forced and not beautiful singing.
On the contrary, if you write too low for them, they might not sound well and basically you would have a hard time hearing them.
Although the range of each individual voice differs from author to author, you can follow these simple guidelines:
Soprano: (b) c1-c3
Alto: (f) g-g2
These are the extremes of each voice, of course. For the best results, make sure you write somewhere in the middle for each voice to be comfortable.
BONUS TIP: sing your melody yourself and you will find out if it is comfortable or not.
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