abrunden
to round off
To bring something (e.g., an event, a meal, a text) to a harmonious or successful conclusion.
Ein Dessert rundet das Menü perfekt ab.
A dessert rounds off the menu perfectly.

to round off / round down
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to round off
To bring something (e.g., an event, a meal, a text) to a harmonious or successful conclusion.
Ein Dessert rundet das Menü perfekt ab.
A dessert rounds off the menu perfectly.

'Abrunden' (to round off / round down) is a separable verb. The prefix 'ab-' comes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Ein guter Wein rundet das Essen ab.
A good wine rounds off the meal.
The verb consists of the prefix 'ab-' and the verb 'runden' (to round). The basic idea is to make something 'round', whether it is a sharp edge, an uneven number, or an incomplete experience.
Der Tischler rundet die Ecken des Tisches sorgfältig ab.
The carpenter carefully rounds off the corners of the table.
The object of the verb reveals the meaning. If it is an experience, a text, or a meal, it refers to the conclusion. With numbers and prices, it is about mathematics. With objects that have corners, it is about the shape.
Eine kurze Zusammenfassung rundet den Vortrag ab.
A short summary rounds off the presentation.
In this figurative meaning, 'abrunden' (to round off) is often combined with abstract nouns that describe an event or work. Typical phrases are: 'ein Programm abrunden' (to round off a program), 'einen Besuch abrunden' (to round off a visit), or 'das Gesamtbild abrunden' (to round off the overall picture).
Ein musikalischer Beitrag rundete die Feier ab.
A musical contribution rounded off the celebration.
When rounding a number down, the result is often given with the preposition 'auf' plus the accusative. This structure specifies the value to which it is reduced.
Die Rechnungssumme von 57,83 Euro wurde auf 57,80 Euro abgerundet.
The invoice total of 57.83 euros was rounded down to 57.80 euros.
to lock / to complete
'abschließen' and 'abrunden' can both describe bringing something to an end. 'abschließen' mainly says that something is finished or ended. 'abrunden' also says that the ending fits well and makes the whole thing feel harmonious. That is why 'abschließen' is used more for projects, training programs, or tasks. 'abrunden' fits better with texts, events, conversations, or meals when a nice ending is missing or is added. The confusion arises because both words refer to a conclusion, but 'abrunden' emphasizes the quality of the ending more strongly.

Ein kurzer Ausblick hat den Vortrag gut abgerundet.
A short outlook rounded off the presentation well.
Vor dem Termin möchte ich den Bericht noch abschließen.
Before the appointment, I still want to finish the report.