Altar 意為「祭壇;聖壇」,恆做名詞。由這個字所構成的兩個成語不時可見,其一是 “to sacrifice something on the altar of something else”,意為「為得到某事物 而犧牲某事物」,如 Robert sacrificed his family life on the altar of his ambition and greed. (羅伯特犧牲了家人的生命來滿足他的野心和貪婪);其二是 “to lead somebody to the altar”,意為「娶某人為妻」,We saw John smiling through his tears as he led Mary to the altar yesterday. (我們昨天見證了約翰淚中含笑地娶瑪麗為妻)。
Alter 意為「改變;修改 (衣服)」,除了在 alter ego (意為「第二個自我 (指通常不為人知的個性的另一面)」,或意為比較罕見的「摯友,密友」) 這個片語中當名詞外,都是當動詞用,如 I will never alter my mind. (我絕不會改變我的主意);The tailor altered this pair of trousers to make it shorter. (裁縫把這條褲子改短了)。
這兩個同音異形異義詞有時會被搞混,尤其是 alter 會被用來取代比較不常見的 altar,如 He was 8 years old at the time and an alter boy. (當時他八歲,是神父的祭壇助手) - 句中 alter boy 為 altar boy 之誤。