Proto-Indo-European 根词，表示“向前”，并通过扩展表示“在前面，之前，首要，朝向，靠近，反对”等。
它构成了以下单词或部分单词： afford; approach; appropriate; approve; approximate; barbican; before; deprive; expropriate; far; first; for; for-; fore; fore-; forefather; foremost; former (adj.); forth; frame; frau; fret; Freya; fro; froward; from; furnish; furniture; further; galore; hysteron-proteron; impervious; improbity; impromptu; improve; palfrey; par (prep.); para- (1) "alongside, beyond; altered; contrary; irregular, abnormal; " paradise; pardon; paramount; paramour; parvenu; pellucid; per; per-; percent; percussion; perennial; perestroika; perfect; perfidy; perform; perfume; perfunctory; perhaps; peri-; perish; perjury; permanent; permeate; permit; pernicious; perpendicular; perpetual; perplex; persecute; persevere; perspective; perspire; persuasion; pertain; peruse; pervade; pervert; pierce; portray; postprandial; prae-; Prakrit; pre-; premier; presbyter; Presbyterian; preterite; pride; priest; primal; primary; primate; primavera; prime; primeval; primitive; primo; primogenitor; primogeniture; primordial; primus; prince; principal; principle; prior; pristine; private; privilege; privy; pro (n.2) "a consideration or argument in favor; " pro-; probably; probe; probity; problem; proceed; proclaim; prodigal; produce; profane; profess; profile; profit; profound; profuse; project; promise; prompt; prone; proof; proper; property; propinquity; prophet; prose; prostate; prosthesis; protagonist; Protean; protect; protein; Proterozoic; protest; proto-; protocol; proton; protoplasm; Protozoa; proud; prove; proverb; provide; provoke; prow; prowess; proximate; Purana; purchase; purdah; reciprocal; rapprochement; reproach; reprove; veneer。
它是假设的源头，其存在的证据由以下提供：梵语 pari “周围，通过”， parah “更远的，远离的，远大的”， pura “以前，之前”， pra- “之前，向前，向前”； 阿维斯塔语 pairi- “周围”， paro “之前”； 赫梯语 para “在外面”，希腊语 peri “周围，关于，靠近，超出”， pera “横跨，超出”， paros “之前”， para “从旁边，超出”， pro “之前”； 拉丁语 pro “之前，为了，代表，而不是”， porro “向前”， prae “之前”， per “通过”； 古斯拉夫语 pra-dedu “曾祖父”； 俄语 pere- “通过”； 立陶宛语 per “通过”； 古爱尔兰语 ire “更远”， roar “足够”； 哥特语 faura “之前”，古英语 fore（prep。）“之前，在前面”，（adv。）“之前，以前”， fram “向前，从”， feor “到很远的地方，很久以前”； 德语 vor “之前，在前面”； 古爱尔兰语 air- 哥特语 fair-，德语 ver-，古英语 fer-，强调前缀。