Generally speaking Dao and not single or double handed, while most are orientated for single handed use I have seen numerous examples of these swords with elongated hilts to facilitate a two handed grip. Wile a lot of Chinese martial arts these days seem to prefer single handed swordsmanship in times of old two handed swordsmanship was equally common especially in the military.
There are countless variations of shape and types of Dao. The four most common are;
Yanmaodao (goose quil saber) – th blade is straight for almost the entire length except for the tip which has a slight curve (the curvature varies).
Luiyedao (willow leaf dao) – the blade has a curvature through out the intire length.
Paindao (slashing saber) – blade shape is similar to Luiyedao exapt that the curvature is far greater it is a very specialized blade and due to its curvature is almost impossiblr to use for thrusting. It is often difficult to distinguish these blades from Luiyedao.
Niuweidao (oxtail dao) – mostly used by militia and commoners not the military got its name from the shape of the blade.
Some of the other styles include;
Miaodao/Changdao (sprout leaf saber) – bears a resemblance in shape and size to the Japanese Odachi.
Zahnmadao (horse killing saber) – use as an anti calalry weapon designed with the idea of killing both horse and rider with one swing in mind.
Qijiadao (Qi style saber) – this Katanaish saber was used by Qi and his forces
Han dao – is said to the the direct ancestor of al other dao and Katana
Zibidao – also refered to as “tang dao” and “chokuto” in Japanese.
WW2 Dadao*
Bagua Dadao* -an overside Dao normally an oxtail saber used in Baugazahn training
Qwandao
Ming Pheonix wind saber – similar to the WW2 dadao but ith a single handed grip it also was developed before th WW2 sword.
Huyadao (tiger tooth saber) – similar to the dadao but with a blade to hilt ratio of 1:1
Tajidao
Shuang Dao- can is basically any type of dao or Jian pair with half hilts and stored in the same scabbard.
Baat Jaam Do (eight chopping knives)
There are others not mentiond here,
*Dado literally means “big knife” and can be used to describe a number of different weapons.
Duan Dao is used wjen reffering to short version of dao.
Some of the swords pictured are just wushu swords ans I had to google the images, I don't know how to post images from my computer.