Hi clem,There are two ways to consider c=1. One is to use units like light years and years so that
speed has the dimensions of LY/Y and c happens to equal 1 in those units.
A more fundamental use of c=1 is that space and time are in the same units so that c=1 and is dimensionless. This is accomplished by giving space and time each the unit of second or each having the unit cm with a conversion between cm and seconds of
1=3X10^10 cm/sec. This is similar to the conversion 1 mile=5280 feet in an American topological map. Similarly with hbar. When hbar and c are 1, a useful conversion is
1=197 MeV-fm.