Traditional US understanding of the Tanka form is as follows.

• a 5-line syllabic poem
• syllable pattern: 5-7-5-7-7

Often, I'll see the subject matter dealing with nature. There's also can be a bit of a volta between the 3rd and 4th lines.

As with the Haiku, the syllable count and line breaks are a bit of a mistranslation.

The line breaks in traditional Haiku and Tanka are arbitrary. A web search for either form will result in a number of different configurations. It's possible to even find them written as a single line: a Haiku of 12 characters and a Tanka of 26 characters.

Written Japanese uses a logographic writing system versus alphabetical. A single Kanji character can encompass a whole concept where a single syllable could only be part of a concept. Here's an example:

交 can mean mix, intersect, exchange/communicate, or deliver. That one symbol can have a syllabic equivalent of up to 4 syllables.

For my purposes, I forgo the line breaks; but keep the syllables. 

For more information about the Tanka form, start with https://en.wikipedia.org/wiki/Tanka

Also, see my project Philadelphia Tanka.