I believe the Royal Law or Law of Liberty is loving your neighbor as yourself.
James 2:8-13
8If ye fulfil the royal law according to the scripture, Thou shalt love thy neighbour as thyself, ye do well:
9But if ye have respect to persons, ye commit sin, and are convinced of the law as transgressors.
10For whosoever shall keep the whole law, and yet offend in one
point, he is guilty of all.
11For he that said, Do not commit adultery, said also, Do not kill. Now if thou commit no adultery, yet if thou kill, thou art become a transgressor of the law.
12So speak ye, and so do, as they that shall be judged by the law of liberty.
13For he shall have judgment without mercy, that hath shewed no mercy; and mercy rejoiceth against judgment.
Romans 13:8-10
8Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law.
9For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if
there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself.
10Love worketh no ill to his neighbour: therefore love
is the fulfilling of the law.
Matthew 22:36-40
36Master, which
is the great commandment in the law?
37Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.
38This is the first and great commandment.
39And the second
is like unto it, Thou shalt love thy neighbour as thyself.
40On these two commandments hang all the law and the prophets.