abstimmen
to vote
To make a decision or express one's opinion through an election or by casting a vote.
Das Parlament wird morgen über das neue Gesetz abstimmen.
Parliament will vote on the new law tomorrow.

to vote / coordinate / tune
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to vote
To make a decision or express one's opinion through an election or by casting a vote.
Das Parlament wird morgen über das neue Gesetz abstimmen.
Parliament will vote on the new law tomorrow.

'Abstimmen' is a separable verb with the prefix 'ab-'. In a main clause, 'ab' goes at the end; in a subordinate clause, it remains attached to the verb. The infinitive with 'zu' is 'abzustimmen'.
Die Mitglieder stimmen heute über den Vorschlag ab.
The members are voting on the proposal today.
The prepositions and the grammar help distinguish the meanings. 'Über' (+ accusative) signals a vote, 'auf' (+ accusative) means to adapt or coordinate something with something, and 'sich mit' (+ dative) means to coordinate with someone.
Wir stimmen über den neuen Plan ab.
We are voting on the new plan.
When it is about an election or a vote, the preposition 'über' is always used with the accusative. You vote 'über' a topic or a proposal.
Die Bürger konnten über den Bau des neuen Bahnhofs abstimmen.
The citizens were able to vote on the construction of the new train station.
To express that you adapt or coordinate something with something else, use the structure: 'etwas (Akkusativ) auf etwas (Akkusativ) abstimmen' (to coordinate something with something).
Der Koch stimmt die Gewürze genau auf die Hauptzutat ab.
The chef coordinates the spices precisely with the main ingredient.
When people coordinate with one another, the verb becomes reflexive and is used with the preposition 'mit' and the dative: 'sich mit jemandem abstimmen' (to coordinate with someone).
Bitte stimmen Sie sich mit Ihren Kollegen über den Termin ab.
Please coordinate with your colleagues about the appointment.
to be right / to vote / to tune
'stimmen' and 'abstimmen' overlap in two areas. For decisions, 'stimmen' means that a person casts their vote for or against something. 'abstimmen' more often describes the whole process in which a group decides by voting. With musical instruments, both can also mean adjusting them to the correct notes; in everyday language, 'stimmen' is especially common for this. The words are easy to confuse because they sound similar and both have to do with votes or voices. For an individual vote, people usually say 'für etwas stimmen' (to vote for something); for the shared decision-making process, 'über etwas abstimmen' (to vote on something).

Nach der Diskussion stimmt die Klasse über das Ziel der Reise ab.
After the discussion, the class votes on the destination for the trip.
Ich stimme für den Vorschlag, weil er fair ist.
I am voting for the proposal because it is fair.