What is gerundival attraction?

Gerundival attraction is what happens when a gerund needs to take a direct object - something grammatically incorrect in Latin (a gerund cannot take a direct object).For example, the phrase 'I reply by writing a letter' 'respondeo epistulam scribendo' is not grammatically correct in Latin.In order to convey the same meaning, we use gerundival attraction:The direct object is 'attracted' to the case of the gerund e.g. epistulam scribendo > epistula scribendo (acc. noun > abl. noun)The gerund is 'attracted' to the number and gender of the direct object e.g. epistula scribendo > epistula scribenda (nt. sg. gerund > fem. sg. gerund)We now have the grammatically correct sentence: 'respondeo epistula scribenda' 'I reply by writing a letter'n.b. 'scribenda' now looks very like a gerundive of obligation and if you translate it as a gerundive, the sentence still means more or less the same thing - 'I reply with a needing-to-be-written letter'. What has actually happened here is that we have turned the gerund into a gerundive so that it can take a direct object.