Linguists call a single unit of meaning, whether "tree" or "UN -", a morpheme, and some languages bind them together obligatorily.

语言学家把一个语义单位称作一个词素，如“tree(树)”，或“un -(前缀“否”)”。一些语言将词素结合在一起。

Human Demodex is a permanent parasitic mite that is obligatorily harbors in hair follicles and in pilosebaceous glands of human skin, especially human facial skin.

人体蠕形螨是专性寄生在人体毛囊和皮脂腺的一种永久性寄生螨，以面部为主。

Due to limitation of pages, the "obligatorily read" content of the local newspapers is reduced and a few newspapers have to add pages, but the proportion of news is decreasing.

地方报纸由于版面所限，“必读性”内容减少，少数报纸不断扩版但新闻比重不断降低。