A system comprising: a first accelerometer; a first fastening article attached to the first accelerometer, the first fastening article capable of holding the first accelerometer in place on a portion of a thumb of a user; a second accelerometer; a second fastening article attached to the second accelerometer, the second fastening article capable of holding the second accelerometer in place on a portion of a wrist of the user, where the wrist and the thumb are of a same hand of the user; and a processing device configured to perform operations comprising: receiving a first set of acceleration measurements from the first accelerometer; detecting, based at least in part on the first set of acceleration measurements, when the thumb of the user wearing the first fastening article and the second fastening article has been tapped against a surface; receiving a second set of acceleration measurements from the second accelerometer; when a tap of the thumb is detected, determining, based at least in part on the first set of acceleration measurements and the second set of acceleration measurements, which tap target from among a set of tap targets located on fingers of the hand of the user has been tapped by the thumb; mapping the tap of the tap target to a symbol associated with the tap target; and transmitting, storing or displaying the symbol. a first accelerometer; a first fastening article attached to the first accelerometer, the first fastening article capable of holding the first accelerometer in place on a portion of a thumb of a user; a second accelerometer; a second fastening article attached to the second accelerometer, the second fastening article capable of holding the second accelerometer in place on a portion of a wrist of the user, where the wrist and the thumb are of a same hand of the user; and a processing device configured to perform operations comprising: receiving a first set of acceleration measurements from the first accelerometer; detecting, based at least in part on the first set of acceleration measurements, when the thumb of the user wearing the first fastening article and the second fastening article has been tapped against a surface; receiving a second set of acceleration measurements from the second accelerometer; when a tap of the thumb is detected, determining, based at least in part on the first set of acceleration measurements and the second set of acceleration measurements, which tap target from among a set of tap targets located on fingers of the hand of the user has been tapped by the thumb; mapping the tap of the tap target to a symbol associated with the tap target; and transmitting, storing or displaying the symbol. receiving a first set of acceleration measurements from the first accelerometer; detecting, based at least in part on the first set of acceleration measurements, when the thumb of the user wearing the first fastening article and the second fastening article has been tapped against a surface; receiving a second set of acceleration measurements from the second accelerometer; when a tap of the thumb is detected, determining, based at least in part on the first set of acceleration measurements and the second set of acceleration measurements, which tap target from among a set of tap targets located on fingers of the hand of the user has been tapped by the thumb; mapping the tap of the tap target to a symbol associated with the tap target; and transmitting, storing or displaying the symbol.