「その方向へ」、「付加」を意味します。派生した英単語には attend や append があります。attend は「参加する」を意味します。参加するには開催地「へ向かい」ます。append は「付加する」を意味します。付加は「同じ方向へ」付け加えていくことです。

ad- から生まれた英単語を見ていると、ad- は、対象への方向性を付加する傾向があるように思えます。

ad- のつづりは ac-、af- などと多彩に変わります。変わりかたにはある程度の規則があります。accept の場合は、ad- に続く発音が cept で、cept の先頭の音は [k] であるため、ad- は ac- になりやすいです。affect の場合は、af- に続く発音が fect で、fect の先頭の音は [f] であるため、ad- は af- になりやすいです。

ラテン語 ad が由来です。ad の語源は印欧祖語 *ad- だとされます。

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