Which do I say?
You have good handwriting.
or
You have a good handwriting.
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Sign up to join this communityWhich do I say?
You have good handwriting.
or
You have a good handwriting.
Handwriting is usually uncountable, and is almost always uncountable in everyday conversation. If you are complimenting someone's handwriting, you would say
You have good handwriting.
However, handwriting can be countable if you are talking about a class or general group of text that contain many different styles. For example, in Joseph B. Lightfoot's Essays on "Supernatural Religion", he discusses "seventh or eighth century handwritings":
In some handwritings of the seventh or eighth century, where the letters have a round form, the substitution of OT for EG would be far from difficult.
The plural is appropriate here because the handwritings of the 7th and 8th centuries are varied and contain many different forms.
If you don't know whether a noun is countable, a good resource is Wiktionary. In its entry for handwriting, it clearly states:
handwriting (usually uncountable, plural handwritings)
handwriting is uncountable.
handwriting noun [ U ]B1 writing with a pen or pencil: We need to ensure that handwriting is properly taught in our primary schools.
Good is an adjective while well is an adverb answering the question how. Sometimes well also functions as an adjective pertaining to health. Reference
As handwriting is a noun, we use good, an adjective.
You have good handwriting.