Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)A classical necessary condition for an ordered set of n + 1 functions F to be an ECT-system in a closed interval is that all the Wronskians do not vanish. With this condition all the elements of Span(F) have at most n zeros taking into account the multiplicity. Here the problem of bounding the number of zeros of Span(F) is considered as well as the effectiveness of the upper bound when some Wronskians vanish. For this case we also study the possible configurations of zeros that can be realized by elements of Span(F). An application to count the number of isolated periodic orbits for a family of nonsmooth systems is performed. (C) 2016 Elsevier Inc. All rights reserved.4481171186FAPESP [2015/02517-6, 2015/24841-0, 2016/11471-2]MINECO/FEDER [MTM2013-40998-P, MTM2016-77278-P, UNAB13-4E-1604]AGAUR [2014SGR568]European Community [FP7-PEOPLE-2012-IRSES-316338, FP7-PEOPLE-2012-IRSES-318999]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)