Examples from literature

• He now felt not disinclined to take up the neglected billet-doux. He broke the seal and read. 
• I sent her three or four billets-doux every day, and received as many. 
• Nothing but billets-doux were found in his writing-case. 
• The sexton who emptied it of its contents found in it more than fifty sequins, and several billets-doux, to the great scandal of the weaker brethren. 
• This billet-doux was sealed and despatched, and in due time brought an acceptance.