 There are also no official figures for the total number of Linux systems, partly due to the difficulty of quantifying the number of PCs running Linux (see Desktop Linux#Measuring adoption), since many users download Linux distributions.Hence, the sales figures for Linux systems and commercial Linux distributions indicate a much lower number of Linux systems and level of Linux adoption than is the case; this is mainly due to Linux being free and open source software that can be downloaded free of charge.A Linux Counter Project had kept track of a running guesstimate of the number of Linux systems, but did not distinguish between rolling release and standard release distributions.