You need to spend some time learning about styles.

Basically, all content in Word is formatted according to the styles applied to it plus some local formatting changes which might be applied over the top of the styles. The attributes of each style can be customised so that "Normal" in one document might be Arial 12 but in another document it is Calibri 12. If text formatted in that style is pasted from doc A to doc B then the font will automatically change since its formatting comes from the style attributes.

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Andrew Lockton
Chrysalis Design, Melbourne Australia