Concerning and regarding are related to topicality, with and without to having. The prepositions plus, minus, and times are related to mathematical operations. Than and as signify comparison. And so forth. Morphology Given that English prepositions hardly ever inflect, discussion of their morphology is generally limited to word formation. English prepositions are formed through both derivation and compounding, and some carry inflectional morphology associated with other parts of speech. Derivational morphology Most prepositions are simple bases consisting of a single morpheme, like in, from, and under. Historically, however, English prepositions have been formed from the prefixes a- and be-. This a- prefix originally contributed the meaning "on" or "onto" but is no longer productive; that is, it is no longer used to form new words. The preposition aboard, for example, can be paraphrased as "on board of". The be- prefix originally meant "about" but, in prepositions, came to mean something closer to "at" or "near". For example, one sense of the preposition before means "at or near the front". Though the be- prefix is still productive in forming words of certain parts of speech, it is no longer used to form new prepositions. Some morphological bases of prepositions remain productive. For example, -wards, which occurs in prepositions such as afterwards and towards, can attach to other morphemes to form new prepositions such as sun in we looked sunwards. Inflectional morphology Despite lacking an inflectional system of their own, English prepositions occasionally carry inflectional morphemes associated with other parts of speech, namely verbs and adjectives. For example, some English prepositions derive from non-finite verb forms and still carry the associated inflectional affixes. The prepositions barring and concerning, for example, contain the -ing suffix of present participle verb forms. Similarly, the prepositions given and granted contain, respectively, the -en and -ed suffixes of past participle verb forms. The prepositions near and far are unusual in that they seem to inflect for comparison, a feature typically limited to adjectives and adverbs in English. Compound prepositions A compound preposition is a single word composed of more than one base. Often, the bases of compound prepositions are both prepositions. Compound prepositions of this kind include into, onto, throughout, upon, within, and without. Compound prepositions have also been formed from prepositions and nouns. Compound prepositions of this kind include some transitive prepositions, such as alongside, inside, and outside, but they are typically intransitive, such as downhill,