WSC Q99: The Rule of Prayer

Q. 99 What rule hath God given for our direction in prayer?

A. The whole word of God is of use to direct us in prayer; but the special rule of direction is that form of prayer which Christ taught His disciples, commonly called The Lord's Prayer.

Does it seem strange to talk about a rule for prayer? Isn't real prayer supposed to be spontaneous and free? It is to be a sincere expression of our desires, but it is also to be according to God's will.