A forumite asked me off-line for information and pictures of Nagamaki; I thought there might be interest by others as well regarding this topic, so here we go:
The two major Japanese pole arms are the Yari (a double-edged spear with a more or less straight blade), and the Naginata (spears with curved, single-edged blades). Naginata are thought to have been used since the Heian period. Their size and shape reflects that of Tachi of each period, and generally in the Muromachi period they were rather short with a broad Monouchi area and very deep Sakizori curvature, rather like weapons of the Nambokuch˘ period.
In times of war the Naginata was used in battle, and it was the favorite weapon of the S˘hei, the warrior-monks. In the Edo period the Naginata became the preferred weapon of women, and for instance the female guards of the ďoku, the "harem" of the Sh˘gun, were armed with Naginata.
Nagamaki is a term that, strictly speaking, only applies to a special style of mounting a Naginata. It was in vogue from the middle of the Muromachi period until the Momoyama period. "Nagamaki" means "long wrap", and refers to the fact that the shaft is wrapped like a sword hilt. Another term used is "Nagadachi" (long Tachi), and some argue that it's actually only a variant of the Nodachi, with longer hilt and shorter blade in comparison.
Although Nagamaki is basically only a style of mounting, there are certain characteristics that make a Naginata a Nagamaki even without the wrapping.
The shaft is rather short, about four feet long, and often there's a Tsuba. The blade is usually longer than a Naginata, has less curvature, and is basically a Sh˘bu-zukuri Katana. Although the blade is constructed like a broad, heavy Katana, the Mune is sometimes thinned along the back to reduce weight, thus giving the blade a more pronounced diamond cross-section.
In the peaceful Edo period many Naginata and Nagamaki were modified to be worn as swords, the so-called Naginata-naoshi and Nagamaki-naoshi respectively. While most Naginata were made into Wakizashi, Nagamaki were often long enough to being converted into a Katana. However, with many Sasuga (shortened tip section - the Hamon runs straight out at the Mune without a Kaeri) blades it's hard to tell whether it originally was a Naginata or Nagamaki.