0-6-6-0

First known tank engine version
First known tank engine version
First use	1916
Country	Germany
Locomotive	Saxon Class XV HTV
Railway	Royal Saxon State Railways
Designer	Heinrich Lindner
Builder	Sächsische Maschinenfabrik

Equivalent classifications
Equivalent classifications
UIC class	(C)C
French class	030+030
Turkish class	33+33
Swiss class	3/3+3/3
Russian class	0-3-0+0-3-0

0-6-6-0
	Baltimore and Ohio’s Old Maude of 1904, the first 0-6-6-0 Mallet in the United States
UIC class
Equivalent classifications
UIC class	(C)C
French class	030+030
Turkish class	33+33
Swiss class	3/3+3/3
Russian class	0-3-0+0-3-0
First use
First known tank engine version
First use	1916
Country	Germany
Locomotive	Saxon Class XV HTV
Railway	Royal Saxon State Railways
Designer	Heinrich Lindner
Builder	Sächsische Maschinenfabrik
First use
First known tender engine version
First use	1904
Country	United States of America
Locomotive	Class O Old Maude
Railway	Baltimore and Ohio Railroad

Under the Whyte notation for the classification of steam locomotives, a 0-6-6-0 wheel arrangement refers to a locomotive with two engine units mounted under a rigid locomotive frame, with the front engine unit pivoting and each engine unit with six coupled driving wheels without any leading or trailing wheels. The wheel arrangement was mostly used to describe Mallet locomotive types and in some occasions, Double Fairlie locomotives.

A similar wheel arrangement exists for Double Fairlie, Meyer, Kitson-Meyer and Garratt locomotives, but on these types it is referred to as 0-6-0+0-6-0 since both engine units are pivoting.