The syntax for designing low pass filters is :
b = fir1(n,Wn,'ftype',window);
n is the number of coefficients, Wn is the cut-off frequency ( 0<=Wn<=1), 'ftype', 'ftype' accepts parameters 'high','stop','DC-1' and 'DC-0'. If the 'windows' parameter is not specified , then Hamming windows is the default option.