1 When because follows a negative construction the meaning can be ambiguous. In the sentence he did not go because he was ill, for example， it is not clear whether it means either 'the reason he did not go was that he was ill' or 'being ill wasn't the reason for him going; there was another reason'. Some usage guides recommend using a comma when the first interpretation is intended （he did not go， because he was ill） and no comma where the second interpretation is intended， but it is probably wiser to avoid using because after a negative altogether. 2 As with other conjunctions such as but and and, it is still widely held that it is incorrect to begin a sentence with because. It has， however， long been used in this way in both written and spoken English (typically for rhetorical effect)， and is quite acceptable. 3 On the construction the reason ... is because, see usage at REASON.