standard / repeatedly
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standard
Included as part of the standard equipment of a mass-produced product (especially vehicles), meaning at no extra cost.
Bei diesem Automodell ist die Klimaanlage serienmäßig eingebaut.
In this car model, the air conditioning is installed as standard.
The word consists of 'die Serie' (the series) and the ending '-mäßig', which means 'according to' or 'in the manner of'. So it describes something that corresponds to the series or happens as if in a series.
Die serienmäßige Ausstattung des Wagens ist bereits sehr gut.
The standard equipment of the car is already very good.
The meaning depends strongly on the context. In connection with products (especially cars), it means 'standard'. In connection with actions (often in sports or everyday life), it means 'repeatedly' or 'one after another'.
Das Navigationssystem ist bei diesem Modell serienmäßig.
The navigation system is standard on this model.
In the meaning of 'standard equipment', the word can be used as an adjective before a noun or as an adverb. Both forms are very common.
Die serienmäßigen Felgen gefallen mir nicht.
I don't like the standard rims.
In the meaning of 'repeatedly', 'serienmäßig' (repeatedly) is almost always used as an adverb that describes an action. Here it often has a slightly negative or ironic tone.
Er verpasste serienmäßig die einfachsten Bälle.
He repeatedly missed the easiest balls.
regular
'regelmäßig' and 'serienmäßig' can both have to do with repetition, but they are usually used in different situations. 'regelmäßig' describes that something happens again and again or at regular intervals. 'serienmäßig' often means that something is already part of the standard equipment of a product, especially with cars or devices. It can also mean that something happens repeatedly in the same way, but that is more specific. Learners may confuse the words because both sound connected to sameness and repetition. For habits and appointments, use 'regelmäßig'. For standard equipment in products, 'serienmäßig' is the right word.
Das Auto hat eine Klimaanlage, die serienmäßig eingebaut ist.
The car has air conditioning installed as standard.
Meine Nachbarin fährt regelmäßig mit dem Zug zur Arbeit.
My neighbor regularly takes the train to work.