Щ, the Russian cursive letter shcha
Щ is the twenty-seventh letter of the Russian alphabet and stands in Russian for a long soft hushing sound, though several other Cyrillic alphabets read it as a two-part cluster. In handwriting it belongs to the group of letters built from one repeated element.
Easy to confuse with
Looks like ш (Cyrillic sha)
Ш and Щ are named together, with И, Л and М, as letters whose cursive forms make some Russian words a challenge, and both draw on the same repeated element. Shape alone will not settle it: work out which reading gives a real word in the sentence, and compare the form against other places in the same document where the writer has used each letter unambiguously.
Looks like и (Cyrillic i)
Both letters consist, entirely or in part, of the element identical to the dotless Latin cursive ı, and combinations of these letters cannot be deciphered unambiguously without knowing the language or having broader context. Treat a run of identical strokes as a list of candidates rather than a reading, and let the word decide; whole words can coincide, as мщу and лицу do.
History
Shcha, also called Shta or Scha, is a letter of the Cyrillic script. In Russian it represents the long voiceless alveolo-palatal fricative /ɕː/, similar to the sh of sheep. Elsewhere it stands for a cluster: in Bulgarian it is /ʃt/ and the letter is named Shta, while in Ukrainian and Rusyn it is /ʃt͡ʃ/, a hard sh followed by ch, as in borscht. That cluster value preserves the historical character of щ as a combination of sounds, consistent with its Old Church Slavonic origins, and Belarusian went further and abolished the letter in favour of the phonetic spelling шч. In English it is romanised as shch, ŝ, šč or occasionally sch; learners are often told to pronounce a cluster, which no longer matches standard modern Russian.
Handwriting is where щ costs a reader time. It is one of the letters built, entirely or in part, from the element identical to the dotless Latin cursive ı, alongside и, л, м, ш and ы, and some words become genuinely undecidable because of it: мщу, meaning I avenge, and лицу, the dative of face, are absolutely identical in cursive form. Russian words may also pose a challenge through the similarity of Ш, Щ, И, Л and М.
Russian cursive was developed during the 18th century on the base of skoropis, the 14th to 17th century chancery hand descended from the earlier Cyrillic bookhands ustav and poluustav, and became the handwritten counterpart of the civil, or Petrine, printed script. Every scribe had his own hand and his own peculiarities of forming and grouping letters, so unclear places are read by comparing them with parts of the same document already deciphered.