  This paper studies balancedness for infinite words and subshifts, both for
letters and factors. Balancedness is a measure of disorder that amounts to
strong convergence properties for frequencies. It measures the difference
between the numbers of occurrences of a given word in factors of the same
length. We focus on two families of words, namely dendric words and words
generated by substitutions. The family of dendric words includes Sturmian and
Arnoux-Rauzy words, as well as codings of regular interval exchanges. We prove
that dendric words are balanced on letters if and only if they are balanced on
words. In the substitutive case, we stress the role played by the existence of
coboundaries taking rational values and show simple criteria when frequencies
take rational values for exhibiting imbalancedness.
