What do we pay our tithing on? The links below represent various different Latter-day Saint views (and some Christian ones) that seek to answer that question –
Four part series on Tithing from LDS Anarchist –
Part 1 | Part 2 | Part 3 | Part 4
Tithing on Gross or Net?
Thoughts from the Great and Spacious Building
Tithing and the Poor
Commentary from By Common Consent
Also some Questions on Tithing they covered
Tithing, Barter & Consumption
From the Splendid Sun blog
A Modern Look at Tithable Income (Sunstone)
HTML version | PDF version
Quinn’s overview of Tithing history in Mormon Hierarchy – volume 2
Tithing and Residual Income thoughts
A renegade Mormon shares his views
An ex-Mormon perspective on Surplus
Some general Christian views –
D&C 119 is the revelation that defines tithing –
1 Verily, thus saith the Lord, I require all their surplus property to be put into the hands of the bishop of my church in Zion,
2 For the building of mine house, and for the laying of the foundation of Zion and for the priesthood, and for the debts of the Presidency of my Church.
3 And this shall be the beginning of the tithing of my people.
4 And after that, those who have thus been tithed shall pay one-tenth of all their interest annually; and this shall be a standing law unto them forever, for my holy priesthood, saith the Lord.
5 Verily I say unto you, it shall come to pass that all those who gather unto the land of Zion shall be tithed of their surplus properties, and shall observe this law, or they shall not be found worthy to abide among you.
6 And I say unto you, if my people observe not this law, to keep it holy, and by this law sanctify the land of Zion unto me, that my statutes and my judgments may be kept thereon, that it may be most holy, behold, verily I say unto you, it shall not be a land of Zion unto you.
7 And this shall be an ensample unto all the stakes of Zion. Even so. Amen.
See also –
JST Gen 14:39 Wherefore, Abram paid unto him tithes of all that he had, of all the riches which he possessed, which God had given him more than that which he had need.
Alma 13:15 And it was this same Melchizedek to whom Abraham paid tithes; yea, even our father Abraham paid tithes of one-tenth part of all he possessed.