2 chapter I. An overview of morphology

Conclusion
In these words the second forms are marked: "asks" is a verb in the third person singular which is expressed by the inflection "s". In its counterpart there's no marker like "s" but the absence of the marker also has grammatical meaning: it means that the verb "ask" is not in the third person, singular number.
Such morphemes are called "zero". In the second example the adjective "higher" is in the comparative degree, because of the "- er" while its counterpart "high" is in the positive degree, the absence of the marker expresses a grammatical meaning, i.e. a zero marker is also meaningful, therefore it's a zero morpheme.
There are cases when there's a marker which has not a concrete meaning, i.e. there's neither lexical nor grammatical meaning like: statesman. The word consists of three morphemes: state - s - man.
The first and third morphemes have certain meanings. But "s" has no meaning though serve as a connector: it links the first morpheme with the third one. Such morphemes are called empty. Thus empty morphemes are those that have form but no content.
In contemporary English there are cases when two forms express one meaning like:
He is writing a letter
Two morphemes in this sentence "is" and " - ing" express one meaning: a continuous action.Such morphemes are called discontinuous.
THE LIST OF USED LITERATURE

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