dot the i's and cross the t's

Be meticulous and precise, fill in all the particulars, as in Laura had dotted all the i's and crossed the t's, so she wondered what she'd done wrong . This expression presumably began as an admonition to schoolchildren to write carefully and is sometimes shortened. William Makepeace Thackeray had it in a magazine article ( Scribner's Magazine, 1849): "I have . . . dotted the i's." [Mid-1800s]